fix(app): stabilize initial timeline rendering

[Brendonovich]

Summary

• defer timeline Markdown until converted HTML is applied, retaining the last rendered HTML while streaming
• include orphan leading-turn history enrichment in initial readiness instead of prepending rows after reveal
• render cached complete timeline pages immediately while their refresh finishes
• hide cold pinned timelines until mounted Markdown and virtual row measurements settle
• reveal cold timelines with a one-time reduced-motion-aware fade
• isolate follow, overflow, and jump state per session before virtualizer remounts
• clear entrance animation classes after completion so later layout changes cannot replay the fade

Validation

• bun typecheck (packages/session-ui)
• bun test src --only-failures (packages/session-ui): 95 passed
• bun typecheck (packages/app)
• bun test --conditions=solid --only-failures --preload ./happydom.ts src/session/timeline/model.test.ts: 7 passed
• cached and cold session-tab smoke tests: 2 passed
• bunx playwright test e2e/regression/session-timeline-history-root.spec.ts e2e/regression/session-timeline-transport.spec.ts: 10 passed
• bunx playwright test e2e/regression/session-timeline-tool-projection.spec.ts: 6 passed
• live Desktop CDP probe: one timeline node and stable row range throughout fade
• push hook: full monorepo typecheck