refactor(studio): gate Acorn writer migration

[jrusso1020]

What

Put the Recast-to-Acorn writer migration behind an executable capability and parity gate.

Why

Acorn lacked motion-path operations and still depended on Recast-only hold synchronization after mutation.

How

Generate the operation matrix, implement missing Acorn paths and shared hold behavior, differentially test semantics/source placement, and keep the flag gated until graduation.

Test plan

• Acorn capability report, motion-path, parser parity, and studio-server writer tests
• Stack-wide lint, format, build, typecheck, and relevant integration gates

[jrusso1020]

• refactor(repo): resolve changed-code audit #2174
• test(engine): isolate peer abort regression #2173
• refactor(cli): decompose render phases #2172
• refactor(cli): centralize process lifecycle #2171
• fix(cloudrun): publish adapter contract #2170
• test(repo): execute packed subpaths #2169
• refactor(repo): centralize package subpaths #2168
• refactor(core): own edit protocol contract #2167
• test(compiler): enforce preview render parity #2166
• refactor(studio): gate SDK operation families #2165
• refactor(studio): gate Acorn writer migration #2164 👈 (View in Graphite)
• refactor(producer): make capture plans immutable #2163
• refactor(producer): unify render requests #2162
• refactor(engine): type capture failures #2161
• refactor(engine): manage child process lifecycles #2160
• refactor(engine): add fingerprinted browser leases #2159
• refactor(render): centralize execution ownership #2158
• refactor(core): unify composition contract #2157
• fix(studio): enforce optimistic file concurrency #2156
• fix(studio): make sdk cutover transactional #2155
• fix(render): publish artifacts atomically #2154
• fix(render): surface structured outcomes #2153
• fix(player): own connection and media resources #2152
• fix(core): preserve runtime transport contract #2151
• fix(player): version runtime protocol #2150
• test(repo): verify packed consumers #2149
• test(repo): enforce workspace contracts #2148
• fix(core): enforce strict runtime safety #2147
• test(producer): gate source tests by execution lane #2146
• fix(core): make timing masks safe for source loaders #2145
• chore(repo): forbid tracked generated artifacts #2144
• main

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[jrusso1020]

Fixed the fresh-head CI compile failure on head 0038842: atomic split batches now resolve the selected GSAP writer once at the route boundary and pass it through each split mutation, matching regular mutation batches. Validated with files.test.ts (66/66), Studio Server typecheck, diff check, and commit hooks. Fresh exact-head CI is running.

[jrusso1020]

Fixed the Windows-only CI failure on head bb7820d: Windows Core compilation types process.env without the selector field, so both GSAP route boundaries now pass an explicit HYPERFRAMES_GSAP_WRITER value instead of the entire environment. Behavior is unchanged. Validated files.test.ts (66/66), Studio Server typecheck, Core build, diff check, and commit hooks. Fresh exact-head CI is running.

[miguel-heygen]

The regular mutation route now selects the writer explicitly and returns a clean 400 for an invalid flag, and the new Acorn motion-path differential coverage is strong. The capability matrix is also compile-time exhaustive.

Blocker

• packages/studio-server/src/routes/files.ts:2045 — foldAtomicCutFile dispatches the split with the selected writer, but after a changed script it unconditionally calls loadGsapParser() and runs the Recast hold synchronizer. Therefore HYPERFRAMES_GSAP_WRITER=acorn atomic split batches still import and execute Recast, violating this migration gate's stated no-Recast runtime invariant and mixing writer semantics in one mutation. The current capability test only scans applyGsapMutations, so this sibling path is not covered. Please use the same writer-conditioned hold synchronization as the regular route and add an atomic Acorn regression proving the Recast import gate is untouched.

Verdict: REQUEST CHANGES
Reasoning: The primary Acorn route and parity suite look solid, but the atomic split path still crosses into Recast under the Acorn flag; the migration gate is incomplete until that path and its regression coverage are corrected.

— Magi

[jrusso1020]

Fixed the atomic Acorn split-path blocker on exact head 8dd7e3d. After a split mutation changes the script, hold synchronization now follows the selected writer: Acorn uses the Acorn synchronizer and never loads Recast; the default Recast path is unchanged. Extended the writer capability regression to cover foldAtomicCutFile and assert the same conditioned hold-sync boundary. Validation passed: files + capability suites 70/70, Studio Server typecheck, formatting, diff check, and commit hooks. The full stack was restacked and republished; fresh exact-head CI is running.

[miguel-heygen]

Fresh exact-head delta review at 8dd7e3dfc.

• packages/studio-server/src/routes/files.ts:2045 now keeps post-split hold synchronization writer-conditioned: Acorn stays on syncPositionHoldsBeforeKeyframes, while the unchanged default path lazily loads Recast.
• packages/studio-server/src/routes/gsapMutationCapabilities.test.ts:47 now includes foldAtomicCutFile in the migration-boundary regression, pinning both the Acorn and Recast branches.

The prior atomic-path blocker is resolved. I rechecked the one-commit delta, all existing reviews/comments and threads, exact-head required CI, mergeability, and the surrounding writer plumbing; no remaining blocker found.

Verdict: APPROVE
Reasoning: The Acorn atomic split path is now Recast-free under the flag, the default Recast behavior is preserved, and regression coverage plus all required exact-head checks are green.

— Magi