fix(pwndbg): reject core mapping paths that escape sysroot

Interpret source-bound ELF64 NT_FILE descriptors once before debugger
launch. Bound counts and pathname coverage by the descriptor, retain exact
path bytes, and refuse relative names or dot segments rather than letting
GDB resolve them against the host. Pin GDB's pathname semantics to Unix.

Preserve the decoded partial observation on refusal, basic-only recovery,
and pre-admission cancellation. Share an exact mapping fixture and replace
the old fixture boolean modes with explicit report producers.

Validation: final series has 33 passing Vitest cases plus five Python
producer controls; 38 Linux x64 provider cases are explicitly skipped on
macOS. Typecheck/lint, docs checks and test discovery pass. Real GDB16.3
and strace reproduce the relative-path escape using only an owned fixture;
compiled admission admits a real absolute-path core and refuses that
relative-path core. Full native pwndbg / Linux x64 remain unverified here.
Task token, CPU and peak-family-memory costs are unmeasured; no speed claim.

Fixes #4942

docs/recorded-crashes.md

@@ -61,9 +61,21 @@ The outer provider owns both processes and one stable core snapshot. An owned
 Python launcher lowers inherited soft limits before replacing itself with GDB.
 GDB starts without initfiles, auto-load, debuginfod or automatic shared-library
 loading. Owned empty library search paths and caches are used, with plugin
-updates disabled. Fixed package code verifies a core connection and no paired
-executable before querying mapping candidates. No caller GDB command language is
-exposed. The exact observed GDB/plugin version strings are retained.
+updates disabled. Debug-file lookup is also restricted to the owned empty
+search directory. GDB 16 can otherwise locate the recorded executable independently
+of these settings. While opening the core, fixed package code temporarily selects
+`/proc/self/exe` (the already running debugger) with `exec-file`, without loading
+symbols or running an inferior. This occupies GDB's executable slot and prevents
+its automatic executable discovery. The validated decoder profile selects the
+core architecture explicitly, so the placeholder cannot select it instead. The
+bridge discards that selection with `file` before querying mapping candidates,
+and verifies the core connection and absence of a main symbol file. Before launching GDB, the provider also checks
+source-bound `NT_FILE` descriptors: mapping names must be absolute Unix paths
+without dot segments. Relative names bypass GDB's sysroot, and traversal can
+escape it. Unresolved or unsafe mapping coverage refuses debugger enrichment
+while retaining the basic decoded observation and original bytes. No caller GDB
+command language is exposed. The exact observed GDB/plugin version strings are
+retained.
 
 Mapping names are producer display metadata, with unknown current file identity.
 Map permissions are derived from reported flags; zero/unfamiliar flags leave
@@ -93,4 +105,6 @@ GDB/Python/pwndbg/strace. Fixture generation explicitly launches its owned test
 program; subsequent inspection only reads the resulting core. Real acceptance
 checks recorded threads, high registers, missing/malformed notes, opaque owners,
 CLI/MCP parity, optional debugger context and historical PID collision with an
-owned live sentinel. Original inputs and host configuration remain unchanged.
+owned live sentinel. The syscall trace also rejects access to the still-present
+source executable. The CI workflow defines Ubuntu 24.04 and Debian 13 (GDB 16)
+container lanes. Original inputs and host configuration remain unchanged.