refactor(session-projection): compact checkpoint row fields to ver/seq/val

The persisted row (sessionId, key, stateVersion, observedSeq, state)
becomes (sessionId, key, ver, seq, val) — the cache medium repeats these
three names for every unit of every session, so the long forms dominated
the JSON payload. ProjectionCheckpointRow and the checkpointRow zod spec
rename together; the domain spec bumps to v3 (cache semantics: the old
medium is discarded, not migrated). The unit-facing declaration keeps
stateVersion — only the persisted/checkpoint row shape changes.

packages/session-projection/session-projection/README.md

@@ -23,7 +23,7 @@ Session-projection seam. It owns `ctx.sessionProjections`, the registry that DRI
 - **Same-reference means no work.** `apply` MUST return the same state reference for events that do not concern the unit; the drive gates the change feed on `Object.is`, so non-matching events cost one call and nothing downstream.
 - **Whole-value event rule (load-bearing).** A state-carrying log event MUST carry the complete post-change state, never a bare delta — it keeps every transition trivially cheap and every served value self-describing (last-wins for consumers).
 - **Synchronous unit discipline.** `init`/`apply`/`view` MUST be synchronous; carriers read `snapshot()` in the same tick as their page slice, which is what makes `asOfSeq` one consistent cut. An accidentally-async `view` returns a Promise, which fails the boundary `schema.parse` loudly.
-- **State is plain JSON, `stateVersion` is its invalidation anchor.** The persisted projection cache (a later phase) stores `(sessionId, key, stateVersion, observedSeq, stateJson)` rows; bump `stateVersion` whenever the state shape or the fold semantics change so stale rows are discarded instead of forward-applied into garbage.
+- **State is plain JSON, `stateVersion` is its invalidation anchor.** The persisted projection cache stores `(sessionId, key, ver, seq, val)` rows; bump `stateVersion` whenever the state shape or the fold semantics change so stale rows are discarded instead of forward-applied into garbage.
 - **No wire vocabulary here.** The registry exposes only the change feed and the snapshot read face; carriers (api-proxy) mint their own frames (`session/projection`) and blocks from them.
 - **Optional seam.** Domain plugins register under `ctx.inject(['sessionProjections'], …)` so headless assemblies without the registry stay unaffected; carriers use `ctx.get('sessionProjections')` and omit their block/frames entirely when the registry is absent.
 
@@ -43,5 +43,5 @@ None; projections never assemble or send provider requests.
 
 - **Every tail page carries every registered key** — there is no per-key opt-out or lazy-key request shape yet; acceptable while values are UI-scale whole states (a todo list, a goal snapshot), revisit if a domain's value grows large.
 - **Eager drive touches every unit per event** — cheap by construction (whole-value rule, same-reference gate), but a hot path would justify per-unit event-type prefilters, addable without contract change.
-- **The persisted projection cache is a later phase** — cells live in memory only; a restart rebuilds by folding the in-memory log on first touch. The `stateVersion` field is the forward-declared invalidation anchor for that phase.
+- **Registry cells live in memory only** — a restart rebuilds by folding the log on first touch; compositions that mount `dsh-session-projection-cache` seed that fold from persisted rows instead.
 - **Synchronous unit discipline is only partially mechanical** — the boundary `schema.parse` rejects a Promise-returning `view`, but an `apply` that blocks or reads torn non-session state is a review concern; the invariant companion documents why no runtime check exists.