Add phased implementation plan (docs/implementation.md)

21 phases from clean-slate to MVP, dependency-ordered with each
phase ending in a runnable demo state. Includes:

- Pre-implementation audit (the 75-min asset checks from memory.md)
- Phases 0–20 with checklist + acceptance criteria + spec-doc refs
- Two DECIDE points (Phase 13 big-room UX, Phase 19 onboarding)
- Out-of-scope list (procgen, biomes, butchering, surgery, etc.)
- Scope-cut levers ranked by gameplay-cost-per-week-saved
- De-risking spikes per phase
- Wired into memory.md index + session log; current Status =
  Phase 0 (gated by the 75-min audit)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

memory.md

@@ -14,8 +14,9 @@ A 2D, tile-based **cute-farming-RPG-meets-colony-sim** — Rimworld DNA, Going M
 | [`docs/architecture.md`](./docs/architecture.md) | Tech — pawn AI / job system, time/tick model, Godot 4 engine layout, TileMap split, all subsystems (mood, lighting, rooms, hauling, production, combat, storyteller) |
 | [`docs/ui.md`](./docs/ui.md) | Touch UX — work-priority matrix, stockpile/container screens, mood/lighting/rooms cues, combat banners, storyteller event UI, screens still to design |
 | [`docs/art.md`](./docs/art.md) | Owned assets (ElvGames bundle primary, Ventilatore secondary), license, autotile gotcha, audit list, candidates kept for record |
+| [`docs/implementation.md`](./docs/implementation.md) | **Phased build plan** — 21 phases (audit → P0 scaffold → … → P20 export). Checklists, acceptance demos, scope-cut levers, de-risking spikes. Track progress here. |
 
-When working on a feature, read `memory.md` first, then the relevant `docs/` file(s).
+When working on a feature, read `memory.md` first, then the relevant `docs/` file(s). For "what do I build next?" check the **Status** row at the top of `docs/implementation.md`.
 
 ## Decisions & rationale
 
@@ -156,6 +157,7 @@ Same scope as locked in `~/claude/ideas/rimlike/plan.md`. Realistic timeline 3
 - Updated `docs/architecture.md` perf assumptions (60² → 80² with 120² ceiling).
 - `docs/ui.md` got a new **World view camera (locked)** section; removed the world-view bullet from "Screens still to design".
 - Open: auto-roof big-room UX (added to TODOs above) — the ≤8-cell BFS cap silently fails on bigger rooms; player feedback path needs a decision before EnclosureDetector lands.
+- Wrote `docs/implementation.md` — 21-phase build plan with checkboxes, acceptance demos, scope-cut levers, and de-risking spikes. Status row at the top of that file is the canonical "where are we now" pointer; current value is **Phase 0 — Project scaffold & foundations** (gated by the 75-min pre-implementation audit). Wired into the index table above.
 
 ## External references