Chore: roadmap v0.10.x shift + docs pruning

Insert two new version series after v0.9.x:
- v0.10.x Panels + Composable Layout (kernel primitive)
- v0.11.x Notes Reference Extension (first-party extension)

Renumber existing planned work:
- v0.10.x Reference Extensions → v0.12.x
- v0.11.x Sidecar Tier + Polish → v0.13.x

Add new design docs for panels and notes v0.11.x series.
Update v1.0.0 gate criteria and design decisions log.
Remove stale docs (AUDIT-surfaces, USABILITY-SURVEY).
Fix dangling reference in DESIGN-shell-contract.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/DESIGN-shell-contract.md

@@ -5,7 +5,7 @@
 ## Problem
 
 Extension surfaces render into `#extension-mount` with no shell chrome.
-A full audit (docs/AUDIT-surfaces.md) found: 3/4 primary surfaces lack a
+A full audit (v0.7.0 surface audit) found: 3/4 primary surfaces lack a
 notification bell, 2/4 lack a user menu, 4 different topbar implementations,
 user menu never updates on package/role changes, toast-and-forget error
 handling, and empty states that explain nothing.