[G2] Node SDK and MCP packaging with the relayhistory backend: single native addon, `IngestOptions.source`, platform matrix, bundle smoke test

[willwashburn]

Part of #553 (group 2 — depends on burn #557 backend; parallel with #558/#561).

Problem

@relayburn/sdk is a napi-rs facade (crates/relayburn-sdk-node → packages/sdk-node, prebuilt @relayburn/sdk-<platform> packages) and @relayburn/mcp sits on top of it. The Node surface exposes no reader/parser API (packages/sdk-node/src/index.d.ts is verbs only: ingest, writePendingStamp, summary, hotspots, …), and IngestOptions is { sessionId?, harness?: 'claude-code'|'codex'|'opencode', ledgerHome? } — it does not expose IngestRoots, so nothing in JS needs to change semantically. What changes is the build:

• The ai-hist Rust crate links statically into relayburn-sdk-node, so no second native addon is needed and ai-hist-native (relayhistory's own napi package) must not become a dependency — a JS consumer that also installs ai-hist would otherwise load two SQLite copies against the same DB file.
• Both crates bundle SQLite (rusqlite bundled); one binary, one SQLite.
• The platform matrix in .github/workflows/napi-build.yml / cli-build.yml must still build (glibc floor, musl, Windows — burn Add Windows targets to napi sdk-node + CLI binary distribution #359 is open for Windows).
• pnpm run test:bundle (esbuild smoke) must keep passing.

Scope

1. IngestOptions gains source?: 'builtin' | 'relayhistory' and historyDbPath?: string (maps to StoreOptions.db_path; default = relayhistory's own default path resolution incl. AI_HIST_DB). LedgerOpenOptions gains the same so Ledger.open can pin a backend for the process. writePendingStamp.harness type widened per [G3] Harness expansion through relayhistory: Cursor and Grok in burn (SourceKind, pricing, TOOL_ALIASES, overhead mapping, fidelity policy) and collector-backlog transfer #560.
2. Surface relayhistory's typed Error variants (SyncLocked, WatermarkAheadOfStore, SourceMismatch, database-open failure) as @relayburn/sdk error classes with codes, alongside the existing ledger errors.
3. Build: enable the relayhistory-source feature in the napi and CLI release builds (feature stays off for the plain cargo build until [G3] Cutover: relayhistory becomes the default and only source; delete the harness readers, walkers, cursors and fs watcher; ship as a major release #562); verify binary size delta and startup time; document in packages/sdk-node/CHANGELOG.md.
4. CI: extend napi-build.yml/cli-build.yml so every platform artefact runs burn --ledger-path <tmp> ingest with RELAYBURN_SOURCE=relayhistory against a corpus fixture HOME and asserts appendedTurns > 0; extend verify-publish.yml accordingly.
5. @relayburn/mcp: no tool changes; add a test that burn mcp-server over a relayhistory-backed ledger returns the same summary as the builtin ledger (fixture).
6. Docs: packages/sdk-node/README + index.d.ts JSDoc for the new options; state that @relayburn/sdk does not depend on ai-hist-native.

Acceptance

• npm ls in a project with both @relayburn/sdk and ai-hist shows no shared native package; both open the same ai-history.db sequentially without error (WAL, read-only in burn).
• All platform artefacts pass the ingest smoke under the relayhistory backend.
• pnpm run test and pnpm run test:bundle green.

Out of scope

Default flip (#562).