[P2] A Studio edit heals every legacy row except a flow's — saveMetaItem re-persists the pre-protocol dialect an author was served #4542

Follow-up to #4498 (PR #4504), verified empirically before filing. Probe: boot the real stack, seed one pre-17 flow (delete_record + config.filters) and one pre-17 object (conditionalRequired, retired #3855), read each via getMetaItem, edit only the label, save back exactly what was served.

Measured result

Two seams compose into the asymmetry:

1. Read: getMetaItem canonicalizes every type through convertStoredItem — which returns flow untouched (the ADR-0078 conflict-guard skip). Studio is served the pre-protocol dialect.
2. Write: saveMetaItem's schema gate accepts it back — FlowNodeSchema.config is an open z.record, so config.filters is not off-spec, just legacy. No conversion runs on the write.

So the implicit contract "an author's explicit save leaves the row canonical" — which is exactly what the once-per-boot stored-conversion warning promises for every other type ("re-save it (Studio edit → save …) to persist the canonical shape") — holds for everything except flows. A flow row stays pending in the --stored report no matter how many times an author edits it; only the migration itself can retire it.

Why the write gate is the right seam (not the read)

The obvious-looking fix — canonicalize flow reads via resolveFlowCanonicalizer (#4498) — is the wrong one:

• Reads are hot and list-shaped; canonicalizeStoredFlow runs a full parse + cycle detection + control-flow validation per body.
• It throws on an invalid flow, and the read path's contract is availability over purity ("reads diagnose, never drop") — a broken row must stay readable and fixable in Studio.
• Execution is already covered: registerFlow canonicalizes at the load seam.

The write gate is where ADR-0087's premise ("new rows are always canonical") lives, and it is the same seam #4498 already fixed for duplicatePackage. saveMetaItem is the one remaining producer-shaped path that accepts a legacy flow dialect — both on the Studio roundtrip and for any API caller that authors config.filters directly today.

Sketch

In saveMetaItem, for singular === 'flow' with an item present:

1. Resolve via resolveFlowCanonicalizer() ([P2] duplicatePackage still mints pre-protocol flow rows — the "strictly shrinking" premise does not hold for flows #4498 — already lazy, already tested). Absent → save as today: a control-plane host must not start refusing flow writes it accepted yesterday.
2. Canonicalize before the schema gate (the [P2] The spec contract stops at authored source — stored metadata is rehydrated unparsed, unconverted, and ungated #3903 principle: chain-owned legacy shape is lifted, not failed) and persist storable.
3. Conflicts (refused node-type rename over a live custom executor) → refuse the save loudly, naming the token — never persist the un-renamed body. Same posture as duplicatePackage/[P3] os migrate meta --stored skips flow rows — the finish line stops short of the one type that needs the executor registry #4454.
4. Parse throw → fall back to saving the raw item and let today's gate decide. canonicalizeStoredFlow is stricter than the schema alone (cycle detection, region validation); a work-in-progress draft with a temporary cycle must not become unsaveable when it saves fine today.
5. Idempotence keeps the other callers cheap: migrateStoredMetadata and duplicatePackage hand in already-canonical bodies, and the pass is copy-on-write, so the second run changes nothing.

Decision to settle at review: whether step 4's fallback also applies in publish mode or only for drafts — publish-refusing an unparseable flow is defensible (it cannot register anyway), but it is a behaviour change for bodies that save fine today.

Evidence trail

Probe harness identical to meta.stored-flow-resolution.integration.test.ts (real bootSchemaStack, inert automation plugin, SQLite on disk). One probe artifact worth knowing about for whoever picks this up: rows seeded raw carry checksum: null, and saveMetaItem's derived parentVersion then disagrees with the column, so the save 409s — pass an explicit parentVersion in probes. (Real rows written by any governed or loader path carry a checksum; not a production issue.)

Refs #4498 (PR #4504), #4454, #4327, #3903, ADR-0087, ADR-0078.