refactor(sfx): extract shared scene SFX picker into sfx-selector

[nguyentran4896]

Summary

• src/pipeline.ts (lines ~129–169) and rerender.ts (lines ~67–93) contained near-identical loops that iterate over scenes and call the 3-tier SFX selector to build a per-scene SfxMixSpec[] list.
• This PR extracts that loop into a new exported function pickSfxForScenes() in src/assets/sfx-selector.ts and updates both call sites.
• Behavior is fully preserved: same SFX files are picked in the same order. The only difference between the two original loops was logging style; the new function accepts an optional logger parameter so each call site retains its own log style (log.info/log.warn in pipeline, console.log/silent in rerender).

New function signature

pickSfxForScenes(args: {
  scenes: Script["scenes"];
  sceneStarts: Record<string, number>;
  sfxIndex: SfxIndex;
  sfxDir: string;
  logger?: { info(msg: string): void; warn(msg: string): void };
}): SfxMixSpec[]

New test cases (src/assets/sfx-scenes-picker.test.ts)

• Returns empty list when scenes array is empty
• Uses explicit sfx override when the file exists on disk
• Skips scene when sfx.name === "none"
• Picks semantically matched SFX when voiceText triggers a keyword rule
• Falls back to template default when no semantic keyword matches
• Passes log messages to the provided logger
• Skips and warns when explicit SFX file does not exist on disk

Verification

npm run typecheck and npm test both pass (51 tests, 10 test files + new file).

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