port: make the Godot menu shell the agent's primary objective

Sets a new mission: boot the real disc through developer splash -> intro video
-> title -> main menu -> submenus in Godot 4, interactively, with no gameplay,
no 3D and no emulator.

docs/port/MISSION.md defines it -- eight gated milestones, each finished by an
ARTIFACT rather than by compiling, plus the Ready Room as an explicitly gated
stretch goal with a one-iteration probe that decides go/no-go. GP_READY_ROOM is
1106 entries with 6 recoverable names and is ISL-scripted, so it is either a
week or a quarter, and the agent must not start it on its own authority.

Architecture, per the user's decision: the Godot project is INDEPENDENT of the
Rust viewer and never reads a disc format. An offline Rust exporter converts the
disc into open formats; Godot reads only those. No GDExtension, no Rust in the
Godot project, and sylpheed-viewer is off limits -- it stays the human's
verification tool with its static-data rule intact.

docs/port/FORMAT.md specifies the open format, versioned, because modding is the
port's second goal and that makes the layout a deliverable rather than a temp
directory: JSON over XML (Godot parses JSON natively; its XMLParser is SAX),
names never hashes, provenance in every generated file, and unknowns listed
rather than guessed.

The discipline the whole thing rests on is the derived/authored split. `export/`
is regenerated wholesale and never hand-edited; `authored/` is hand-written and
survives a re-export. Three things this milestone needs are NOT on the disc in
any decoded form -- which button does what, paint order, and menu sound cues --
so they live in `authored/` with a stated `why`. Deleting an authored entry
because the exporter can now emit it IS the measure of progress.

`export/` is gitignored: it is generated from the user's own disc and this stays
a clean-room repo.

Container: adds a pinned Godot 4 (windowed under Xvfb for screenshots, plus a
headless wrapper). ffmpeg already carries libtheora, which is the video target --
Godot 4 plays only Ogg Theora natively and the disc's ADV.wmv is WMV3/WMA Pro.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.gitignore

@@ -21,3 +21,7 @@ Thumbs.db
 # Trunk build output
 dist/
 __pycache__/
+
+# The Godot port export is generated from the user's own disc.
+# Code, schemas and authored mappings are versioned; game assets are not.
+export/