recover: the F5/F6 port work from the deleted auto/port-p6-audio

A snapshot of the non-game files as of 0148cb8 ("port: F5/F6 hand-off --
one-minute human checks, and a refutation attempt that survived",
2026-09-04), the tip of auto/port-p6-audio. The branch was deleted from the
server on 2026-09-17 during the consolidation cleanup; issue #7 asks for
this work as a reviewable PR, so it is recovered here before the commits
are garbage collected.

Contents: the 84 files the branch changed relative to its fork point
b305aa4, which is this commit's parent. The tree is therefore 0148cb8's
tree with the 854 exported game assets left out -- export-probe/,
export-probe2/, three .wav renders of game audio and adv-v2-screenlog.tsv.
Game data stays out of git; the exporter regenerates those from the disc.
docs/port/DECISIONS.md still refers to them by name.

Not recovered: the branch's own 366 commits. Keeping them would make those
assets reachable again, so this is one snapshot instead. The original
commits stay unreferenced in the server's object store, and in this clone
under the local branch archive/port-p6-audio, until either is garbage
collected.

Refs #7. The OPTIONS work that issue #6 asks for is a subset of this
branch, also recovered as recover/options-menu.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

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.