webapp: load a replay script from the file system

[macabeus]

The recorder panel could record and replay a script but never open one, so a script saved to disk was write-only. This PR closes the loop.

What's in it

• script-parser.ts — the exact inverse of the serializer. It shares the serializer's button vocabulary (BUTTON_NAMES / buttonsFromString derive from the same BIT_TO_BUTTON_NAME table), so a button name cannot exist in only one direction.
• Strict parsing, per-line errors. An unsupported statement is an error with a line number, not a silent skip — a replay that silently dropped a line would "work" and desync. Comments are blanked in place (offsets preserved) so the reported line numbers match the file the user is looking at.
• Recorder panel: a Load button + error panel; Save and the "From recording start" origin are disabled while no snapshot exists, since both are meaningless before recording begins.
• 27 new tests (57 total), including a round-trip property pinning parse(serialize(segments)) = segments.
• docs/examples/klonoa-hidden-minigame.mjs — a real script to load: it boots Klonoa: Empire of Dreams holding A+B+RIGHT+L+R(+UP) and plays a few dodges of the hidden boot-menu minigame.

Verification

• pnpm test (57), check-types, lint, check-deps all clean.
• Exercised end-to-end in a browser: loading the example script replays the minigame, and loading a file with bad statements shows the per-line errors instead of replaying a truncated script.

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