feat(substrate): LOCK schema — 6-week parallel Stream A/B/C/D window opens

Schema at SUBSTRATE-SCHEMA.md (revised 2026-04-22) is immutable until
~2026-06-03 or explicit user revocation. 4 parallel streams may now
proceed independently against this contract.

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

docs/SCHEMA-LOCKED.md

@@ -0,0 +1,50 @@
+# Substrate Schema — LOCKED
+
+**Locked on:** 2026-04-22
+**Locked from commit:** feat/substrate-schema-v1 @ 5d089bc
+**Schema SSoT:** [SUBSTRATE-SCHEMA.md](./SUBSTRATE-SCHEMA.md)
+
+## Lock scope
+
+The atom / capability / graph contracts defined in `SUBSTRATE-SCHEMA.md` are **immutable for 6 weeks** of parallel Stream A/B/C/D work (through ~2026-06-03).
+
+## What "locked" means
+
+**Non-breaking during lock** (allowed, standard git flow):
+- New atom kinds beyond `command | query | stream | transform`
+- New optional frontmatter fields
+- New `side_effects.op` values
+- New `stability` levels
+- New JSON Schema field examples / constraints that don't break existing validators
+
+**Breaking during lock** (requires revocation):
+- Changing atom ID separator `::`
+- Changing frontmatter shape
+- Renaming `Cargo.toml [package.metadata.keisei]` fields
+- Dropping JSON Schema draft-07 in favour of 2020-12
+- Switching to shared error registry
+- Any change that forces all 4 streams to rebase
+
+## Revocation protocol
+
+To break the lock within the 6-week window:
+
+1. User explicitly states: "revoke substrate schema lock — reason: <...>"
+2. All 4 stream agents paused
+3. Schema edit committed on `feat/substrate-schema-v2` with revision log
+4. All 4 stream worktrees rebased onto new schema
+5. Ledger row: `kei-ledger fork <id> --reason "schema-revocation"` for audit
+6. New lock marker committed
+
+## Streams under lock
+
+Per Stream interfaces in SUBSTRATE-SCHEMA.md:
+
+- **Stream A** — `feat/stream-a-kei-forge` — web wizard UI that generates atoms
+- **Stream B** — `feat/stream-b-atoms-refactor-kei-task` — pilot refactor of `kei-task` into 3 atoms
+- **Stream C** — `feat/stream-c-kei-sage-substrate` — kei-sage walks `atoms/*.md`, resolves wikilinks
+- **Stream D** — `feat/stream-d-kei-runtime` — new crate `kei-runtime` with `invoke`, `list-atoms`, `kei-schema-lint`
+
+## Unlock condition
+
+Automatic unlock on 2026-06-03 OR on `SCHEMA-UNLOCKED.md` commit by user (whichever comes first). Post-unlock, the next schema version (`SUBSTRATE-SCHEMA.md` v2) can iterate freely based on what the 4 streams learned.