Consolidate the core engine into a frameengine/ package

[diego3]

Summary

Physically consolidates every package this repo's docs/frame_engine_migration_plan.md
classified as Move into a single new top-level frameengine/ directory, so that extracting
diego3/frame-engine later is a straight copy instead of
another round of figuring out what goes where.

Stacked on #26 (the WorldScene/MainMenu split) — that had to land first, since this move
needs to carry the generic scene.WorldScene, not the old game-logic-leaking MainMenu. Marked
draft until #26 merges, to keep the diff here scoped to just the move. Base is
claude/split-worldscene-from-metalslug, not main.

What moved

Every directory the plan's inventory marked Move, with its internal layout preserved exactly
as-is — this directory's structure is the plan's proposed frame-engine repo layout:

frameengine/
├── application/{config,engine,game,data}/
├── event/, events/
├── object/
├── physics/{,box2d}/
├── ports/
├── process/
├── resource/
├── script/
├── vec2/
└── view/{ui,input,camera,scene}/

games/, logic/, main.go, main_wasm.go, and docs/ all stay where they are — per the
plan's inventory, they're the application-layer side of the eventual split, not the engine.

What this is (and isn't)

Pure move + import-path rewrite (goengine/x → goengine/frameengine/x) within the same Go
module — no behavior change anywhere. The only files modified in place (not moved) are the three
outside frameengine/ that import it: main.go, main_wasm.go, and
games/metalslug_demo/scene.go.

When frame-engine is ready to actually receive this code, the extraction becomes: copy
frameengine/'s contents to the new repo's root, go mod init github.com/diego3/frame-engine,
and strip the frameengine/ segment from every import path. No further restructuring needed.

Not done here (deliberately, to keep this PR mechanical and reviewable): moving docs/adr/ and
docs/tdr/ (the plan also marks these Move, but they're prose with their own cross-links,
not import paths — better as its own follow-up), and updating .claude/skills/game-architecture
/game-dev's file-path references (already flagged as a known follow-up in the plan itself).

Validation

• go build ./..., go vet ./..., gofmt -l (clean).
• go test ./... and go test -race ./...: all passing except the same pre-existing
  box2d-go checkptr failure this repo has documented multiple times before (triggered by any
  CreateWorld call, unrelated to this repo's code or this move).
• WASM cross-compile check (GOOS=js GOARCH=wasm go build): passes.
• Live smoke run of both games/demo1 and games/metalslug_demo: identical output to before the
  move (same physics-body listing, same point where the sandbox's known audio limitation stops
  the run).

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