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

docs/private/roadmap/phases/phase-20.9/STRUCTURE.md

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+# Phase 20.9 — Structure (Lang Line)
+
+Decision
+- Introduce `lang/` at the repository root to host the C ABI kernel and related artifacts (no Rust here).
+- Keep Rust engine/crates under `crates/` and `src/` untouched. The `lang` line is a separate box.
+
+Layout
+- `lang/` — top-level
+  - `c-abi/` — headers and libc-backed shim for canaries
+    - `README.md` — responsibilities and contracts
+  - `README.md` — scope and principles
+
+Rationale
+- Clear separation of concerns: script-driven ABI vs. Rust runtime internals.
+- Makes it easy to package/link the C shim with external harnesses without dragging Rust sources.
+
+Notes
+- Migration is incremental: existing headers/shims remain in `include/` and `c/abi/` until references are updated.
+- CI/docs should point to `lang/c-abi` once stabilized; older paths deprecated after a grace period.