Drive trait schema enums from PreviewTraits canonical lists

[obj-p]

Summary

Step #5 of the architectural-refactor roadmap. The previous shape duplicated the 12-string `dynamicTypeSize` enum verbatim across `previewStart` and `previewConfigure` schemas — adding a new value meant editing `PreviewTraits` plus both schemas, with no test catching a forgotten edit.

After the refactor, both schemas reference `PreviewTraits.validColorSchemes` / `validDynamicTypeSizes` / `validLayoutDirections` / `validLegibilityWeights` directly. Adding a new value touches one place (`PreviewTraits`) and both schemas pick it up automatically.

Changes

• `PreviewTraits.swift`: convert the four `valid*` constants from `Set` to ordered `[String]`. Order matters because the schema enum arrays are wire-relevant (the `ListToolsSnapshotTests` byte-snapshot pins array order in JSON). Internal callers (`.contains` for validation, `.union` in `allPresetNames`) all work on arrays or are wrapped with `Set(...)`.
• New `Handlers/TraitPropertySchema.swift` with a small `traitProperty(enumValues:description:)` helper that builds the consistent `{type, enum, description}` Value shape. Descriptions stay per-handler since `previewStart` and `previewConfigure` legitimately say different things ("override" suffix vs "Pass empty string to clear" suffix).
• `PreviewStartHandler.swift` and `PreviewConfigureHandler.swift`: trait property literals replaced with `traitProperty(enumValues: PreviewTraits.validX, description: ...)` calls.

Acceptance gate

• `ListToolsSnapshotTests` passes — the JSON-encoded `ListTools` response is byte-identical pre/post (the `Tests/PreviewsCLITests/list_tools_snapshot.json` fixture from Split MCPServer.swift into per-tool handler files behind ToolHandler #162 didn't change).
• New `TraitSchemaTests` asserts both handlers' enum constraints match the canonical `PreviewTraits` arrays. Catches the realistic regression: adding a value to `PreviewTraits` but forgetting to pick it up in a schema.

What's NOT changed

• Trait extraction in `parseTraits` (in `MCPServerSupport.swift`) still names the 5 trait fields explicitly. `PreviewTraits.validated` takes 5 named parameters; refactoring that requires a deeper PreviewsCore API change. Out of scope.
• Asymmetric `enum` enforcement: `previewConfigure` deliberately doesn't enforce `enum` on `layoutDirection` / `legibilityWeight` (it accepts empty-string-as-clear). Preserved.

Test plan

• `swift build` clean
• `swift-format format --in-place --recursive Sources/ Tests/PreviewsCLITests/` clean
• `swift test --filter ListToolsSnapshotTests` — wire bytes match
• `swift test --filter TraitSchemaTests` — 3 new tests pass
• `swift test --filter MacOSMCPTests` — 7 tests pass (including `structuredContent payloads decode for each migrated tool` and the variants test)
• `swift test --filter PreviewsCoreTests` — 246 tests pass (including `BridgeGeneratorTraitsTests.validColorSchemes` and `validDynamicTypeSizes` which compare against the now-Array constants)

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