diff --git a/.gitattributes b/.gitattributes
index 6313b56c57..3b11aee0fe 100644
--- a/.gitattributes
+++ b/.gitattributes
@@ -1 +1,6 @@
 * text=auto eol=lf
+
+# TEMPORARY (Torch migration): checked-in archive. text=auto would almost certainly
+# detect this as binary anyway, but an eol conversion on a 4.4 MB zip would corrupt it
+# silently, so say so explicitly. Remove with prebuilt/ -- see prebuilt/README.md.
+prebuilt/soh.o2r binary
diff --git a/.gitignore b/.gitignore
index 987a859c10..c9c30c8a7f 100644
--- a/.gitignore
+++ b/.gitignore
@@ -461,3 +461,6 @@ soh/properties.h
 /clang-format
 /clang-format.exe
 *.o2r
+# TEMPORARY (Torch migration): the checked-in soh.o2r that GenerateSohOtr copies.
+# Remove this negation when Phase 4's packer replaces it -- see prebuilt/README.md.
+!prebuilt/soh.o2r
diff --git a/CMakeLists.txt b/CMakeLists.txt
index d92b7379b2..63b3bb9283 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -244,20 +244,52 @@ add_custom_target(
     DEPENDS ZAPD
 )
 
-# Target to generate only soh.o2r
+# TEMPORARY (Torch migration): soh.o2r is copied from prebuilt/, not generated.
+#
+# The real producer drives ZAPD via OTRExporter/extract_assets.py, and the migration deletes
+# both. Its replacement -- a small in-tree packer, Phase 4 of torch-migration/PLAN.md -- has to
+# reproduce ZAPD's texture quantisation exactly, so rather than block Phases 2/3/5 on it, the
+# archive is checked in and copied into place. See prebuilt/README.md for provenance and for how
+# to restore a real build step (which must also bring back the libultraship shader copy that the
+# copy-only version below has no use for -- the shaders are already inside the archive).
+#
+# Fail at configure time if the prebuilt archive's baked-in portVersion no longer matches the
+# project version. OTRGlobals.cpp:283 demands exact major.minor.patch equality and RunExtract
+# exit(1)s with "soh.o2r is outdated" -- an obscure symptom for what is really a stale checked-in
+# file. portVersion is 7 bytes: endianness, then u16 major/minor/patch big-endian.
+set(SOH_PREBUILT_O2R "${CMAKE_CURRENT_SOURCE_DIR}/prebuilt/soh.o2r")
+if(NOT EXISTS "${SOH_PREBUILT_O2R}")
+    message(FATAL_ERROR "Missing ${SOH_PREBUILT_O2R} -- see prebuilt/README.md")
+endif()
+file(READ "${SOH_PREBUILT_O2R}" _soh_o2r_hex HEX)
+string(FIND "${_soh_o2r_hex}" "706f727456657273696f6e" _pv_at)  # "portVersion" as hex
+if(_pv_at EQUAL -1)
+    message(FATAL_ERROR "No portVersion entry in ${SOH_PREBUILT_O2R} -- see prebuilt/README.md")
+endif()
+math(EXPR _pv_at "${_pv_at} + 22")  # skip the 11-character name
+string(SUBSTRING "${_soh_o2r_hex}" ${_pv_at} 14 _pv)
+string(SUBSTRING "${_pv}" 2 4 _pv_major)
+string(SUBSTRING "${_pv}" 6 4 _pv_minor)
+string(SUBSTRING "${_pv}" 10 4 _pv_patch)
+math(EXPR _pv_major "0x${_pv_major}")
+math(EXPR _pv_minor "0x${_pv_minor}")
+math(EXPR _pv_patch "0x${_pv_patch}")
+if(NOT "${_pv_major}.${_pv_minor}.${_pv_patch}" STREQUAL "${CMAKE_PROJECT_VERSION}")
+    message(FATAL_ERROR
+        "prebuilt/soh.o2r is stamped ${_pv_major}.${_pv_minor}.${_pv_patch} but this build is "
+        "${CMAKE_PROJECT_VERSION}. The game would reject it at startup (\"soh.o2r is outdated\"). "
+        "Regenerate it -- see prebuilt/README.md.")
+endif()
+unset(_soh_o2r_hex)
+
+# Target to place soh.o2r (copy of the prebuilt archive)
 add_custom_target(
     GenerateSohOtr
-    COMMAND ${CMAKE_COMMAND} -E rm -f soh.o2r
-
-    # copy LUS default shaders into assets/custom
-    COMMAND ${CMAKE_COMMAND} -E rm -r -f ${CMAKE_CURRENT_SOURCE_DIR}/soh/assets/custom/shaders/
-    COMMAND ${CMAKE_COMMAND} -E copy_directory ${CMAKE_CURRENT_SOURCE_DIR}/libultraship/src/fast/shaders/ ${CMAKE_CURRENT_SOURCE_DIR}/soh/assets/custom/shaders/
-
-    COMMAND ${Python3_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/OTRExporter/extract_assets.py -z "$<TARGET_FILE:ZAPD>" --norom --custom-otr-file soh.o2r "--custom-assets-path" ${CMAKE_CURRENT_SOURCE_DIR}/soh/assets/custom --port-ver "${CMAKE_PROJECT_VERSION}"
-    COMMAND ${CMAKE_COMMAND} -DSYSTEM_NAME=${CMAKE_SYSTEM_NAME} -DTARGET_DIR="$<TARGET_FILE_DIR:ZAPD>" -DSOURCE_DIR=${CMAKE_CURRENT_SOURCE_DIR} -DBINARY_DIR=${CMAKE_BINARY_DIR} -DONLYSOHOTR=On -P ${CMAKE_CURRENT_SOURCE_DIR}/copy-existing-otrs.cmake
-    WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/soh
-    COMMENT "Generating soh.o2r..."
-    DEPENDS ZAPD
+    COMMAND ${CMAKE_COMMAND} -E copy_if_different "${SOH_PREBUILT_O2R}" ${CMAKE_CURRENT_SOURCE_DIR}/soh.o2r
+    COMMAND ${CMAKE_COMMAND} -E copy_if_different "${SOH_PREBUILT_O2R}" ${CMAKE_BINARY_DIR}/soh/soh.o2r
+    # No BYPRODUCTS: ExtractAssets already claims ${CMAKE_SOURCE_DIR}/soh.o2r as one of its
+    # own (see above), and two rules generating the same path is a hard ninja error.
+    COMMENT "Copying prebuilt soh.o2r (TEMPORARY -- see prebuilt/README.md)..."
 )
 
 if(CMAKE_SYSTEM_NAME MATCHES "Linux")
diff --git a/prebuilt/README.md b/prebuilt/README.md
new file mode 100644
index 0000000000..2b27c59ec4
--- /dev/null
+++ b/prebuilt/README.md
@@ -0,0 +1,87 @@
+# Prebuilt `soh.o2r` — **temporary**
+
+This directory exists to unblock the Torch migration and is **meant to be deleted**.
+
+`soh.o2r` is the port's own asset archive: fonts, textures, presets, translations, shaders —
+everything under `soh/assets/custom/`, plus a `portVersion` stamp. It is built by
+`GenerateSohOtr`, which today drives ZAPD through `OTRExporter/extract_assets.py`. The migration
+deletes ZAPDTR and OTRExporter, so that producer goes away and its replacement — a small in-tree
+packer (Phase 4 of `torch-migration/PLAN.md`) — has to reproduce it byte-for-byte, including
+ZAPD's specific texture quantisation.
+
+Rather than block every other phase on that packer, the archive is checked in here and
+`GenerateSohOtr` just copies it into place. **Phases 2, 3 and 5 become testable without the
+packer existing.**
+
+This file also serves as Phase 4's acceptance oracle: the packer is correct when its output
+matches this archive entry-for-entry. That replaces Gate E's planned sha256 manifest — the
+artifact is strictly more useful than a list of hashes of it.
+
+## Provenance
+
+| | |
+|---|---|
+| Generated | 2026-07-25 |
+| Shipwright branch | `torch-migration` @ `fe2f52248` |
+| `develop` at the time | `c4e92a70f` (inputs verified identical — `git diff develop HEAD -- soh/assets/custom libultraship CMakeLists.txt` was empty) |
+| libultraship submodule | `c57da1b4` |
+| `CMAKE_PROJECT_VERSION` | `9.2.3` |
+| Producer | `cmake --build build --target GenerateSohOtr` (ZAPD `botr -se OTR --norom`) |
+| Size / entries | 4,414,957 bytes / 1,042 entries (1,038 checked-in assets + 3 LUS shaders + `portVersion`) |
+| sha256 | `1d23b582fbc29cebf803b6ee54856089d0978e5496e6d4b1dbcde876e13cf13c` |
+| sha256 of inputs | `de8ba0b5167bec6956258dd0445d01ac0f6daac84aff091c678d951cc72598f6` |
+
+The input hash covers every file under `soh/assets/custom/` except `shaders/`, which is not
+checked in — it is copied out of `libultraship/src/fast/shaders/` at generation time. Recompute it
+with:
+
+```sh
+find soh/assets/custom -type f -not -path '*/shaders/*' -print0 | sort -z \
+    | xargs -0 sha256sum | sha256sum
+```
+
+(`-print0`/`sort -z` are required: several preset filenames contain spaces.)
+
+## Generation is content-deterministic, not byte-deterministic
+
+Two consecutive runs produce archives with **identical payloads for all 1,042 entries** but
+**different whole-file hashes** — the zip stores a modification timestamp per entry. So:
+
+- The sha256 above identifies *this* file, not "any correct archive".
+- **Phase 4's acceptance test must compare entry-by-entry**, never whole-file. A byte-identical
+  archive is not achievable and not the goal.
+
+## What this pins
+
+The archive freezes three inputs. If any changes while this workaround is in place, the checked-in
+copy is stale and **nothing will tell you at runtime** except missing or wrong assets:
+
+1. **`soh/assets/custom/`** — edit an asset and your change is silently ignored. Compare the input
+   hash above.
+2. **libultraship's shaders** — `libultraship/src/fast/shaders/` is copied in at generation time,
+   so a LUS submodule bump can drift the three shaders inside this archive.
+3. **The project version.** `portVersion` here is `9.2.3`. `OTRGlobals.cpp:283` requires exact
+   `major.minor.patch` equality against the build's version, and `RunExtract` calls `exit(1)` with
+   "soh.o2r is outdated" if it fails. A configure-time check in the root `CMakeLists.txt` catches
+   this and tells you to regenerate, because the runtime symptom is not obvious.
+
+## Regenerating
+
+From a tree where ZAPDTR/OTRExporter still build (i.e. `develop`):
+
+```sh
+cmake --build build --target GenerateSohOtr
+cp soh.o2r prebuilt/soh.o2r
+```
+
+Then update the provenance table above.
+
+## Removing this
+
+When Phase 4's packer lands and its output matches this archive entry-for-entry, delete this
+directory, drop the `!prebuilt/soh.o2r` negation from `.gitignore`, and restore `GenerateSohOtr`
+to a real build step (including the libultraship shader copy, which the copy-only version skips).
+
+**Delete it before the migration PR merges** if at all possible. It is a 4.4 MB binary, and a blob
+that reaches `develop` is in the history permanently; removed in the final tree and squash-merged,
+it never lands there at all.
diff --git a/prebuilt/soh.o2r b/prebuilt/soh.o2r
new file mode 100644
index 0000000000..21070d1da1
Binary files /dev/null and b/prebuilt/soh.o2r differ