Golden gate: Linux baselines, canonical save, gate hardening (D521-D532)

Linux reference-frame set for all 21 levels and the cross-platform golden
rule (D521); golden tooling made non-destructive (D524); the gate pins a
canonical in-tree save (D529) and the PRNG seed (D532); the capture tool's
per-level re-pin bug fixed and the Linux set re-captured under the
canonical save (D530, D531); no-arg verify.sh exits non-zero. README and
docs visual/consolidation passes; the 15 MB preview gif replaced by stills.

Refs: D521 D523 D524 D525 D526 D527 D528 D529 D530 D531 D532

docs/porting-notes.md

@@ -1441,6 +1441,10 @@ Some decomp functions write past a local array in edge cases. On the N64, IDO's
 
 Also the read side: D496 (`generate_language_specific_text_for_weapon` strcat'd onto an uninitialised caller stack buffer for 3+ players; init under `AVOID_UB`).
 
+## D25. Cross-platform goldens: a fixed frame stem is only comparable at settled gameplay (D521)
+
+`GE_PCDUMP`/`GE_QUITFRAME` step the port's *sim* frame counter (`port/src/video.c:1406` `++frames`, one per `portRenderGfxTask()` at `port/src/libultra.c:1256`), and retrace messages are dropped at the posting site when the game's queue is nearly full (`port/src/libultra.c:753`), so a slow box stretches wall time without advancing `frames` -- frame stems are comparable, wall time is not. What is NOT comparable is any stem inside the intro flyby, whose settle point is wall-clock paced (D117): on a ~54 fps Linux box the flyby was still running at frame 900 (settles ~830-1130 run to run) while the Windows golden at 900 was settled, which reads as a 92.9% "different scene" delta if reported bare. Tells: over-tol2 % >30% at one stem and 1-7% at the next; phash Hamming in the 80s; a dense dump whose consecutive diffs sit at 80-95% and drop to 1-5% once settled. Never compare across platforms with `GE_DETERM=1` (call-sequenced retraces: stable run to run, different frame -> sim mapping). Before calling a residual band a renderer bug, classify it: a constant ~20 delta confined to a screen band (Archives' top 3 grid rows, all three stems) is systematic and needs a same-sim-state capture pair; a uniform d 3-6 over the whole frame with cell means <= 11 (Cuba) is the dither/quantization noise floor. **RESOLVED 2026-10-05 (D525):** both residual bands were *save state*, not renderer — the pair that closed them was a re-round of the whole linux set with `data/ge007.eep` pinned present (Archives 19.956-24.554% -> 3.590-3.859%, worst cell 50.8 -> 2.31; Streets 1200's 16.0-148.4 cell-mean band -> 3.616-3.983%, worst cell 4.43). The diagnostic tell that separates the two: re-capture one side with the save present and the band collapses to the noise floor, while a renderer defect survives identical sim state.
+
 ## E. Process / method notes
 
 - **Never `exit()` while another thread may be inside the GL driver (D344).**