docs: restore docs/private/roadmap from 7b4908f9 (Phase 20.31)

docs/private/roadmap/phases/phase-20.26/CHECKLIST.md

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+# Phase 20.26 — Checklist
+
+Status legend: [ ] pending, [~] in-progress, [x] done
+
+Phase A — Deprecation + Docs (safe, no behavior change)
+- [x] Deprecate banner when `llvm-inkwell-legacy` path executes (runner/modes/llvm)
+- [ ] One-time compile-time banner when feature is built (optional)
+- [x] Docs scaffold (README/PLAN/CHECKLIST) present
+- [x] CURRENT_TASK updated to Phase 20.26 focus
+
+Phase B — Archive + Feature Gates
+- [x] Move `src/backend/llvm/compiler/**` → `archive/legacy/llvm-inkwell/**`
+- [x] Keep feature wiring via shim (build stays green; feature off by default)
+- [x] Add opt-in smoke to assert deprecate banner
+
+Phase C — Runner Plan (opt-in)
+- [x] Add lang `RunnerPlanBuilder` skeleton (decision only)
+- [x] Add thin adapter in Rust runner (apply plan)
+- [x] Toggle `HAKO_RUNNER_PLAN=1` guarded; defaults unchanged
+  - v0: action ExecuteCore/ExecuteVM を適用（Gate‑C/VM）。既定不変。
+
+Phase D — hakmem (gate only)
+- [x] CLI `--mem {libc|hakmem}` to ENV (`HAKO_C_ABI_ALLOC`)
+- [x] Keep default libc; docs updated; no behavior change（introspection: `hako_allocator_name()` 追加）
+
+Status (2025-10-30)
+- quick: PASS 56 / SKIP 16 / FAIL 0
+- integration-core: PASS 26 / SKIP 0 / FAIL 0
+- Gate‑C/Core: Early routing (JSON→Interpreter) in place; no boot-plan/ny-compiler noise.
+
+Docs/Comms
+- [x] env-variables: reference `--vm-backend`, `HAKO_LLVM_USE_HARNESS`, `HAKO_C_ABI_ALLOC`
+- [ ] migration notes: prefer harness; inkwell legacy; rollback path
+
+Tests/Smokes
+- [ ] quick stays green (default). No noise banners unless path used.
+- [ ] opt-in smoke: inkwell banner visible when feature path chosen