[trissim]

P21: one renderer, typed layout setters

Production source: +55/-107, net −52. Tests: +4/-4, four existing grid setups now use the stylesheet parser; no new tests and none run. No new module.

• Deleted Screen's duplicate render-damage walk. Existing Compositor render/update methods own the deadline, atomic composition and one bound publication. Native synchronous callers use the existing step runner; no second renderer, timer or fallback cache.
• Damage is consumed when output is consumed, not while binding it; Screen's batch/hold checks run before publication.
• LayoutProperty stores the supplied Layout unchanged, with an identity no-op. Only the stylesheet parser calls get_layout to decode external strings. Deleted the setter's decoder/name round-trip.
• Migrated every identified in-tree string setter: Toad's two goal-control setters use layout instances (Toad tests for auto refresh Textualize/textual#696); four Native test setups use CSS parsing. AST search found no layout.name comparisons. CSS serialization still reads the declared name; it does not choose or replace a layout.

Current-main ratchet exits0: .artifacts/p21/typed-layout-final-ratchet.json. AST boundary/source evidence is retained in .artifacts/p21/ and Toad's layout-clock-before.json (880 production/dependency modules, zero parse omissions). Computed aliases are unresolved.

Toad Textualize#696 needs this setter fix to retain WorkspaceChrome behavior. Merge and install them together; #138 remains held/unpinned.

Performance is not declared solved. The7e/eab source-candidate run measured59ms frame p95 and at most one layout start per paint, but exposed Toad geometry bugs. Those and P65 are repaired in Textualize#696; the fresh order requires P32 next and then one installed-build drag capture. Earlier failed series and raw failures are preserved in Toad's canonical performance table. No new source capture or A/B loop is planned.

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