feat(timing): Stage-2 #1a — data-load cost in psx_instr_base_cycles (Δ-validated)

Transcribed from Beetle PS_CPU::ReadMemory: a CPU load adds +2 cycles (LWC2 +1);
stores posted (~0). psx_instr_base_cycles now returns 3 for LB/LH/LWL/LW/LBU/LHU/LWR,
2 for LWC2, 1 else. Flows to BOTH backends via the single-source seam.
Δ GATE (cyc_watch @0x80017FC4, native vs Beetle per-iteration): 46 -> 56 (toward
Beetle 91) — load cost closed ~10 cyc; remaining ~35 = later components (I-cache
fetch, mult/div, GTE, region/absorb refinements). FMV still streams (no regression).
Approximation noted in the header: no scratchpad-free / load-delay-absorb / region
yet (Δ-gated next).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

runtime/include/psx_instr_cost.h

@@ -31,12 +31,32 @@
 extern "C" {
 #endif
 
-/* CPU instruction base cycle cost (execute cycles only; excludes data-access
- * wait states, which the memory path charges). `insn` is the raw 32-bit MIPS
- * word. Stage 1: identity. */
+/* Guest CPU cycle cost for one retired instruction. `insn` is the raw 32-bit MIPS
+ * word. Transcribed from the in-tree Beetle oracle (mednafen psx/cpu.cpp) + psx-spx;
+ * each component verified against Beetle via the cyc_watch DELTA gate
+ * (FAITHFUL_TIMING_PLAN.md §3c). Both backends consume this one function.
+ *
+ * STAGE 2 component #1 — data-memory access (the dominant ~2x gap):
+ *   Beetle PS_CPU::ReadMemory: a normal CPU load adds `lts += 2` (LWC loads +1);
+ *   stores are POSTED (WriteMemory adds no CPU stall); scratchpad reads add 0.
+ *   So: load opcode = 1 (execute) + 2 (data access) = 3; LWC2 = 2; store/other = 1.
+ * NOT YET MODELED (later Δ-gated components, will adjust here / in the mem path):
+ *   - scratchpad-vs-RAM region (scratchpad load should be 1, not 3),
+ *   - the read-after-load "fudge" (+0/2) and load-delay ABSORB (next-insn discount),
+ *   - I-cache instruction-fetch timing, mult/div latency, GTE per-command costs.
+ *   This RAM-load base is an upper bound on the load component (no absorb yet); the
+ *   Δ gate shows whether we under/overshoot and what to add next. */
 static inline uint32_t psx_instr_base_cycles(uint32_t insn) {
-    (void)insn;
-    return 1u;
+    uint32_t op = insn >> 26;
+    switch (op) {
+        case 0x20: case 0x21: case 0x22: case 0x23:   /* LB  LH  LWL LW  */
+        case 0x24: case 0x25: case 0x26:              /* LBU LHU LWR     */
+            return 3u;                                /* 1 execute + 2 data-access */
+        case 0x32:                                    /* LWC2 (GTE load): +1 */
+            return 2u;
+        default:
+            return 1u;
+    }
 }
 
 #ifdef __cplusplus