feat(mac): migrate renderer hot path to AppKit

[SergeSerb2]

Summary

• Migrates the macOS sidebar, chat timeline, Markdown, and measured diff hot paths to native AppKit controllers while keeping AppModel and the existing domain models authoritative.
• Adds renderer-neutral presentation stores, scoped mutation/action boundaries, keyed collection updates, row reuse and height infrastructure, TextKit 2 Markdown, exact scroll anchoring, and deterministic performance probes.
• Makes AppKit the compiled default after Gates A-E passed. SERGECODE_MAC_RENDERER=swiftui remains the immediate process-local rollback. Legacy SwiftUI deletion is deliberately reserved for Gate F after an accepted release soak.

Area

• apps/mac — native macOS app
• apps/windows — Windows desktop app
• apps/mobile — iPhone companion app
• apps/server — backend server
• Shared packages (packages/contracts, packages/shared, packages/client-runtime, …) or relay (infra/relay)
• Build, CI, or release tooling
• Docs

Release size

• size:XS — tiny internal, documentation, or narrowly scoped fix
• size:S — small user-visible fix or contained improvement
• size:M — normal feature or meaningful behavior change
• size:L — broad feature or substantial cross-package change
• size:XL — release-system, architecture, or ecosystem-scale change

Verification

• pnpm run verify --all passes (check + typecheck + tests; adds lint:mobile and the Swift suite)
• pnpm run test:mac passes (required for apps/mac changes)
• Manually verified (describe below)

Verification results:

• Full gate: all five verification steps passed; 0 check errors, all package typechecks/tests passed, 1,104 macOS app tests passed, 28 SidecarKit tests passed, and the mobile native static gate passed.
• Gate D exact 60-second AppKit stream soak: 1,819 deltas / 3,725,312 bytes; pinned p95/p99 8.335/12.431 ms; unpinned 5.031/6.783 ms; offscreen 5.368/7.165 ms; no qualified-phase stalls over 100 ms and zero full reload/group/activity/fallback/anchor failures.
• Gate E no-override seven-scene matrix passed with renderer=appkit. Qualified stream p95 was 7.392 ms pinned, 4.865 ms unpinned, and 4.559 ms offscreen.
• The identical seven-scene matrix passed with SERGECODE_MAC_RENDERER=swiftui and renderer=swiftui, proving the rollback path.
• Local evidence: .artifacts/appkit-gate-d-stream-soak-final-3/evidence.json, .artifacts/appkit-gate-e-default-final-1/evidence.json, and .artifacts/swiftui-gate-e-fallback-final-1/evidence.json.

Screenshots / Recordings

The in-process UIProbe captured all seven deterministic scenes for both the default AppKit path and explicit SwiftUI fallback under the Gate E artifact directories listed above.

Release notes

SurgeCode now uses a native AppKit renderer by default for the macOS sidebar, conversation timeline, Markdown, and diff review, with a tested SwiftUI rollback switch retained during rollout.