docs: v10 roadmap from v9 deep-dive review

[tcconnally]

Deep-dive code review of v9.0.0 (3,069 LOC, 11 modules) and a phased plan to v10.

TL;DR: v9 is healthy — the v8→v9 rewrite closed most classic footguns (filter_mode, main-thread pause cache, typed config fallbacks, bounded mpv terminate, gen counter, token-gated shutdown). So v10 is a next-level release, not a bug-fix.

Six epics

1. Identity Unification — kill the hyperwall_v8.exe/9.0 string drift (~15 refs) and decouple G-Sync isolation from the literal basename before bumping the version. Foundation.
2. 24/7 Reliability & Self-Healing — per-cell stall watchdog (silent freezes currently go undetected), _switching error-path leak fix, memory-aware cache budget, crash-loop guard.
3. Test Harness & CI — mock-only unit tests for _build_url/needs_transcode/retry transitions/config round-trip + GitHub Actions.
4. Multi-Source Walls — per-monitor/per-cell sourcing + scene presets.
5. Backend Abstraction — MediaBackend protocol → Emby + Jellyfin.
6. Web Remote 2.0 & Observability — SSE push, full-auth mode, /health + /metrics.

Sequencing: v10.0 = epics 1+2+3 · v10.1 = 4 · v10.2 = 5 · v10.3 = 6.

Guardrails carried forward: static=true is load-bearing (Emby 4.9.5.0 500 bug), validate against a live server, no speculative behavior, build hygiene.

Full detail with file:line references in ROADMAP.md. This PR adds the roadmap doc only — no code changes.

---

📍 Tracked as GitHub issues (milestone v10)

• Epic 1: Identity Unification (v8→v10 single source of truth) #6 — Epic 1: Identity Unification (foundation)
• Epic 2: 24/7 Reliability & Self-Healing (stall watchdog + fixes) #7 — Epic 2: 24/7 Reliability & Self-Healing
• Epic 3: Test Harness & CI #8 — Epic 3: Test Harness & CI
• Epic 4: Multi-Source Walls (per-monitor sourcing + scene presets) #9 — Epic 4: Multi-Source Walls
• Epic 5: Backend Abstraction (Emby + Jellyfin) #10 — Epic 5: Backend Abstraction (Emby + Jellyfin)
• Epic 6: Web Remote 2.0 & Observability #11 — Epic 6: Web Remote 2.0 & Observability

Sequencing: v10.0 = #6 + #7 + #8 · v10.1 = #9 · v10.2 = #10 · v10.3 = #11
Milestone: https://github.com/tcconnally/hyperwall/milestone/1