fix(sdk): fs adapter flush() tracks in-flight writes; add to T13 contract suite

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How

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• Unit tests added/updated
• Manual testing performed
• Documentation updated (if applicable)

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• feat(sdk): stage 6 — sub-composition scoped ids (F9) #1434
• feat(sdk): stage 5 — export adapter factories from package root #1432
• docs(sdk-playground): accurate README with stage coverage and CanResult #1429
• feat(sdk): stage 4 — canUndo/canRedo, removeElement GSAP cascade, override-set cleanup #1431
• feat(sdk): can() returns CanResult; T4 dispatch-boundary tests #1426
• fix(sdk): fs adapter flush() tracks in-flight writes; add to T13 contract suite #1425 👈 (View in Graphite)
• feat(sdk): file-backed fs adapter + setTiming GSAP-script sync; add sdk-playground #1423
• refactor(core): swap studio-api read path from recast to acorn parser (T6e) #1392
• feat(core): parse-parity suite for acorn parser (T6d) #1370 : 1 other dependent PR (#1379 )
• feat(core): acorn GSAP write path — magic-string offset-splice (T6c) #1369
• feat(core): acorn GSAP read path with T6b differential corpus tests #1368
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