fix(studio): make design edits conflict-safe

[miguel-heygen]

What changed

• Makes GSAP the source of truth for motion-owned transforms. Studio now edits writable GSAP source or stops with a precise Add selector / Unroll / Open Code action; it never reports success after writing competing CSS that GSAP will overwrite.
• Extends static GSAP analysis through safe helper defaults while treating computed selectors, helper/loop-owned motion, and unresolved keyframes as non-directly-editable.
• Makes drag, resize, rotation, grouped transforms, and inspector commits transactional: preflight is atomic, failed persistence rolls back, and stale async failures cannot roll back a newer edit.
• Adds one external-file-change coordinator for Code, Design, timeline, Preview, and SDK flows. It drains pending saves before reload, suppresses only exact Studio write echoes, and blocks reload on conflicts or failed saves.
• Adds a recovery UI with review, copy, download, take external, overwrite, and retry actions. Recovery snapshots survive reload in IndexedDB.
• Adds a regression for text-field Backspace routing across save/selection refreshes.

Reproduction matrix

Report	Result on current main	Change
GSAP x/y owns the same transform channel as Studio drag	Reproduced: GSAP restores its pixel component on seek	Studio edits GSAP when provenance is directly writable; otherwise it blocks before mutating CSS
#2982 external write during pending Studio edits	Reproduced in the file-change/reload path	Pending work drains first; conflicts stop Preview/SDK reload and preserve both versions
#2981 second Backspace deletes the selected layer	Not reproducible on current main using the reported text-field flow	Regression pins both Backspaces to the input and proves a later canvas Backspace still deletes intentionally

Closes #2982. Refs #2981.

Validation

• Studio: 304 files passed, 1 skipped; 3,262 tests passed, 18 todo
• Parsers: 33 files passed; 908 tests passed, 4 skipped, 3 todo
• Focused regression pass after final integration: 17 Studio files / 137 tests and 2 parser files / 35 tests
• Studio and parser TypeScript checks
• Oxlint with warnings denied
• Oxfmt, diff check, filesize, tracked-artifact, large-file, fallow, and commit-message gates

[miguel-heygen]

Superseded by reviewable sub-700 LOC PRs: GSAP stack #2985 -> #2986 -> #2987; independent Backspace regression #2988; external-change stack #2989 -> #2990 -> #2991 -> #2992 -> #2993. GitHub reports the largest incremental diff as 650 changed lines.