port: land the play-tested work, and only that

Takes the port branch up to 77320d5e -- the state the human play-tested on
2026-09-02 -- for SOURCE paths only. Not a branch merge: `auto/port-p6-audio`
is 366 commits and 938 files, and most of that must not land.

WHAT COMES IN (76 files, all human-confirmed working):
  * the logo splash animation. 08ed3dd1 found it: `pose_at` ASSIGNED the settle
    instant instead of clamping to it, so the splash never animated at all --
    and the same bug manufactured a passing harness result, because the harness
    photographed t past the settle. Confirmed by play-test: "cannot notice any
    obvious difference from the actual game."
  * gamepad input -- (A)/(B) bound additively (`ui_accept` ships with NO joypad
    binding), stick latched with hysteresis at the game's own 61% digitise
    threshold. This is what made (A), video-skip and Extras work at all.
  * menu navigation and flow, menu audio, the exporter, the authored
    declarations, and 23 verification tools under tools/port/.

WHAT IS DELIBERATELY LEFT ON THE BRANCH:
  * everything after c0ae460a -- the F5/F6 title-timing investigation, whose own
    tip commit calls itself a "hand-off for one-minute human checks". Unchecked
    by definition; it goes through the new review gate like anything else.
  * the OPTIONS menu work of 2026-09-03. Real, probably good, NOT play-tested.
  * the F1 repeat mechanism, which its own commit calls "deliberately inert".

WHAT MUST NOT LAND, AND WHY THE .gitignore CHANGED:
  545 MB of extracted game content was committed on that branch -- 850 sprite,
  audio and transcoded video files under `export-probe/` and `export-probe2/`,
  plus 246 MB of loose .wav and .tsv at the repo root. This repository's own
  rule, in this file, is "never game content".

  The rule was not missing. It was written, and it was tightened on that very
  branch, with a careful comment explaining why BOTH `export/` and `data/base/`
  had to be listed -- while the exporter was writing to a third name that
  nobody had thought to list. Enumerating names is the thing that failed. So
  the ignore rules now describe the SHAPE: any top-level `export*/`, game media
  by extension, and loose capture output at the root. Verified both ways -- it
  catches all four offenders and ignores nothing currently tracked.

Verified: `cargo check --workspace` clean; all nine GDScript files parse in
project context, with a positive control (an injected syntax error is detected,
3 lines) so the clean result means something. `tools/port/check-all` was NOT
run -- it needs the container, the export tree and a display.

data/mods/README.md

@@ -0,0 +1,57 @@
+# Your mods go here
+
+A mod **replaces a file by shadowing its path**. There is no manifest, no
+registration and no load order: if a file exists here at the same relative path
+it has in the export tree, the game reads yours instead.
+
+```
+export/sprites/title/main_menu/ptbtn01.png     <- what the exporter wrote
+data/mods/sprites/title/main_menu/ptbtn01.png  <- what the game will use
+```
+
+That works for **every** asset kind the port reads — a screen's JSON, a sprite
+PNG, a sound cue, the music bed, a movie — because every read goes through one
+resolver (`port/scripts/export_tree.gd`, `ExportTree.resolve`).
+
+Nothing under `export/` is ever touched, so **re-exporting from your disc is
+always safe**, and *"did I break it?"* is answered by moving your file out of
+this directory.
+
+Point the game somewhere else with `SYLPHEED_MODS=/path/to/tree`.
+
+## The game tells you what you changed
+
+Every file a mod replaces is printed the first time it is read:
+
+```
+mod: sprites/title/main_menu/ptbtn01.png  <-  /work/data/mods/sprites/title/main_menu/ptbtn01.png
+```
+
+A modded run that looked identical to an unmodded one in the log would leave you
+with exactly one debugging tool — delete the mod and try again.
+
+## Try it in ten seconds
+
+Replace the `NEW GAME` label with a magenta block. The size is the original's,
+`203x43`, and nothing here is derived from the disc:
+
+```bash
+mkdir -p data/mods/sprites/title/main_menu
+ffmpeg -f lavfi -i "color=c=0xff00c8:s=203x43" -frames:v 1 -pix_fmt rgba \
+       data/mods/sprites/title/main_menu/ptbtn01.png
+godot --path port -- --menu
+```
+
+Delete the file to put it back.
+
+## Nothing in here is committed
+
+`.gitignore` excludes everything in this directory except this README. That is
+deliberate: a mod is usually an *edited game asset*, and this repository never
+holds game assets — not in `export/`, and not here either.
+
+## One tree, not a stack
+
+Several mods layering over each other would need a load order, and a load order
+needs a rule nobody has asked for yet. Today there is one override tree. If you
+want more, say so rather than assuming the port has an answer.