B1: persistence primitives — reserveMessage + updateContent pipeline ops

[vellum-apollo-bot]

What

First PR in the backend-anchor workstream (B1 of B1→B5). Adds two persistence primitives — reserveMessage and an updateContent pipeline op — with no production callers. Future PRs will adopt them in the agent loop.

Why

We're moving the agent loop toward pre-allocating the assistant message id at LLM-call start, so every SSE event (deltas, tool_use, tool_result, message_complete) can be stamped with a stable, server-minted messageId from the first byte. That lets us delete the frontend's content-match reconcile path entirely and avoid an optimistic→server id swap dance.

To get there without a 1000-file PR, we ship the building blocks first.

What's in this PR

Memory layer (assistant/src/memory/conversation-crud.ts)

• New: reserveMessage(conversationId, role, metadata?) — pre-allocates a row with content: "[]", returns the row. Mirrors addMessage's SQLITE_BUSY/SQLITE_IOERR retry shape, but intentionally skips Qdrant indexing and attention projection (an empty placeholder isn't meaningful for either).
• updateMessageContent was already there; no change.

Pipeline (assistant/src/plugins/types.ts, assistant/src/plugins/defaults/persistence.ts)

• Extends the discriminated PersistArgs / PersistResult unions:
  • reserve → reserveMessage
  • updateContent → updateMessageContent
• Both flow through the default-persistence pipeline terminal, so plugin middleware can observe, redirect, or short-circuit them.

Tests (assistant/src/__tests__/persistence-pipeline.test.ts)

• New test: reserve op produces a row with content: "[]" and the supplied metadata.
• New test: updateContent op overwrites a previously-reserved row, with direct-call parity against updateMessageContent.
• Existing redirect test now exercises the two new variants on the in-memory mock store.

What's NOT in this PR

• Any production caller — agent loop, conversation-routes, etc. are untouched.
• The new SSE event type (assistant_turn_start) and messageId? wire field — that's B2.
• Agent-loop adoption — B3.

Verification

cd assistant
bunx tsc --noEmit            # clean
bunx eslint <touched files>  # clean
bun test src/__tests__/persistence-pipeline.test.ts
# 7 pass, 0 fail, 41 expect() calls

Plan reference

Backend queue (B1 → B5):

1. B1 — Persistence primitives (this PR)
2. B2 — Wire protocol additions: assistant_turn_start event + messageId? on streaming events. Pure additive, no emit sites.
3. B3 — Agent loop adopts pre-allocation: reserveMessage at every LLM call start, stamp every SSE event with the anchor id.
4. B4 — Bump self-hosted daemon floor so frontend can hard-require B3.
5. B5 — Frontend reconcile rewrite consuming the new anchor identity end-to-end.

[vellum-apollo-bot]

Fixes the Test check failure on this PR.

Root cause. mock.module("../memory/conversation-crud.js", () => ({...})) replaces the module wholesale. background-workers-disk-pressure.test.ts transitively imports the new defaults/persistence.ts, which imports reserveMessage from conversation-crud. The mock factory didn't declare that symbol, so Bun raised SyntaxError: Export named 'reserveMessage' not found in module between tests.

Fix. Added a reserveMessage stub to every conversation-crud mock.module site (81 files). Only one of them was failing in CI today, but every other site is one transitive import away from the same crash — same pattern as PR #27459 with resolveQdrantUrl.

Discovered + patched via skills/software-engineering/scripts/mock-module-audit.ts, the workflow encoded in references/unit-testing.md rule #3.