feat: dsh-session-projection-cache — durable projection checkpoints and the cold-read ladder

New package on the domain data form: one session_projcache record per
session (key → {stateVersion, observedSeq, state}), landing beside
workspace.json under the shipped json backend. Write policy: two mandatory
points (turn/end + session disposal) with count/interval throttling between
them (both Config fields required — flush cadence is a deployment choice);
every background write is fail-soft (log + stay stale, self-heal on the
next write or cold read). coldSnapshot(id) runs the read ladder — cached
rows + persistence readFrom from the registry's anchored restore floor +
registry restore + fail-soft write-back — detecting crash-repair-shrunk
logs via the one-below anchor and degrading to a single full re-read.
Mounted in apps/cli/cordis.yml (writeEveryEvents 200 / writeIntervalMs
5000).

packages/session-projection/session-projection-cache/src/invariant.ts

@@ -0,0 +1,35 @@
+/**
+ * Package-owned invariant companion for `@deepseek-ai/dsh-session-projection-cache`.
+ * @module @deepseek-ai/dsh-session-projection-cache/invariant
+ */
+
+/* jscpd:ignore-start */
+import type { Context } from 'cordis'
+import type { InvariantInstaller } from '@deepseek-ai/dsh-invariants'
+
+const PACKAGE_NAME = '@deepseek-ai/dsh-session-projection-cache'
+
+/** Cordis companion plugin name. */
+export const name = 'session-projection-cache-invariant'
+/** Service required before the companion can reserve package ownership. */
+export const inject = ['invariants']
+
+/**
+ * No runtime invariant: the cache's correctness relation (a stored row equals
+ * the registry fold at its `observedSeq`) is only checkable by re-running the
+ * fold over the persisted log — duplicating the implementation rather than
+ * detecting drift — and its staleness is by design (fail-soft writes). The
+ * durable boundary is already schema-validated by the storage-domain layer
+ * on every reopen, and the read ladder's version/watermark guards are proven
+ * by the package spec.
+ */
+const install: InvariantInstaller = () => {}
+
+/**
+ * Register this package's invariant companion.
+ * @param ctx - Cordis context carrying the invariant service.
+ * @returns the installed registration's disposer after setup succeeds.
+ */
+export const apply = (ctx: Context): Promise<() => void> =>
+  Promise.resolve(ctx.invariants.register(PACKAGE_NAME, install))
+/* jscpd:ignore-end */