feat(vdisplay): lifecycle state machine + display state/release API (Stage 1)

Stage 1 of design/display-management.md — the lifecycle core + the display
management surface:

- vdisplay/lifecycle.rs: pure per-slot state machine (Idle/Active{refs}/
  Lingering{until}/Pinned) with acquire/release/expiry/force-release
  transitions. No I/O, no OS types — the platform-neutral distillation of the
  Windows manager's model. Unit + a 200k-iteration seeded property walk
  (no leaks / double-frees / refcount underflow across arbitrary interleavings).
- vdisplay/registry.rs: neutral snapshot/release facade over the per-OS
  lifecycle owners. Windows reads/controls the VirtualDisplayManager; Linux
  keep-alive (a per-session pool) lands in a following increment (needs GPU-box
  validation).
- windows/manager.rs: additive snapshot() + force_release() (no behavior change
  to the on-glass-validated path).
- mgmt: GET /api/v1/display/state (live/kept displays) + POST /api/v1/display/release
  (tear down lingering/pinned now; refuses active). OpenAPI regenerated.
- web console: Virtual displays card gains a live-display list (polled) with
  per-row + release-all buttons and a linger countdown.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/messages/en.json

@@ -68,6 +68,15 @@
 	"display_save": "Save",
 	"display_effective": "In effect",
 	"display_pending_note": "Conflict handling, identity, and layout are stored but not enforced yet — they arrive in later releases.",
+	"display_live": "Live displays",
+	"display_none_live": "No virtual displays right now.",
+	"display_state_active": "Active",
+	"display_state_lingering": "Lingering",
+	"display_state_pinned": "Pinned",
+	"display_release_btn": "Release",
+	"display_release_all": "Release all kept",
+	"display_expires_in": "tears down in {sec}s",
+	"display_sessions": "{count} streaming",
 	"clients_title": "Paired clients",
 	"clients_empty": "No paired clients yet.",
 	"clients_name": "Name",