feat(editor): clamp timeline scroll bounds

[ChrisBeWithYou]

Summary

• add shared timeline scroll-bound math with a small tail past the song end
• clamp wheel pan, middle-drag pan, zoom, seek, playback follow, pending-view restore, and audio duration changes
• add focused JS coverage for short-song pinning, long-song tail bounds, and invalid scroll values

Testing

• node --check screen.js
• node tests/scroll_bounds.test.js
• node tests/snap_options.test.js
• node tests/eof_shortcuts.test.js
• all tests/*.test.js via node

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• Commit unit tests in branch feat/editor-scroll-bounds

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[byrongamatos]

Review pass (Claude Code) — stacked on #58. Clean, well-structured. The pure _editorViewportDurationPure/_editorMaxScrollXPure/_editorClampScrollXPure are correct — NaN/negative → 0, a short song pins to start, a long song scrolls to duration + tail − viewport — and _editorClampScrollX is applied uniformly at every scroll-mutation site (wheel, middle-drag, zoom, seek, follow, pending-view restore, duration change), replacing the old min-only Math.max(0, …) clamps. node --check + all JS tests green (scroll_bounds + snap + shortcut suites). No bug in this PR.

Flags: (1) soft overlap with #46, which also clamps S.scrollX (its _maxScrollX + _jumpCursorTo) at some of the same sites — they'll textually collide there and add two redundant bound helpers, so it's part of the same #46 reconciliation. (2) Inherits the stack-wide drum-shortcut issue (pre-fix #55 base) I detailed on #58 — no new keybindings here, so nothing #59-specific. (Codex rate-limited until ~1 AM — manual + test-backed.)

[byrongamatos]

Codex review pass — found + fixed 2 real bugs in the scroll-bound math I'd missed:

1. Tail hid the start of short songs: _editorMaxScrollXPure returned duration + tail − viewport unconditionally, so a song that already fits (e.g. 6s song / 7s viewport / 2s tail) still exposed 1s of scroll that could hide the beginning behind a blank tail. Now pins to 0 unless the content actually runs past the viewport.
2. No re-clamp on resize: the max scroll derives from canvas width, but resizeCanvas() redrew with the stale S.scrollX — widening the editor could leave the timeline past the new max. Added _editorApplyScrollBounds() before the resize draw.
   Both pushed; +test for the fits-but-tail case; all JS tests green. (The 2nd fix's final Codex re-confirm was cut off by the usage limit resetting, but the change is small and verified manually.)