notion-md: converge the 6 fake NotionMdGateway impls onto a shared harness + de-brittle body-byte tests

Testing-architecture cleanup surfaced by review (deferred from #786 as its own focused pass).

Fake-gateway convergence

The NotionMdGateway fake is re-rolled 6 times (FakeNotion, FakeTreeNotion, four near-identical FakeGateway classes with copy-pasted snapshot()/pull() + walls of Effect.dieMessage stubs). The house pattern already exists: notion-datasource-sync/src/testing/harness.ts makeFakeGatewayHarness(input) + Partial<>-override fixture builders. Converge on it (notion-effect-client also has createMockHttpClient). Lever: adding a gateway method today forces synchronized edits across 6 files.

De-brittle body-byte assertions

~24 full-body .toBe/.toEqual assertions are the re-baseline burden (vs ~19 already-semantic .toContain). Migrate to semanticEquivalent where the body is incidental, and add one golden-file fixpoint over demo/showcase.nmd (currently no snapshot files exist).

Not a CI bug

dt test:* caching is local-only (execIfModified globs only ${pkg}/src/**; CI does not persist devenv task state — runs cold). P1 developer-trust hygiene, not a stale-green CI risk.

Well-designed models to copy: the live integration harness, the OTEL span-shape tests (with leak guards), and notion-effect-clients provision-once mock layer.

Start with notion-datasource-sync/src/testing/harness.ts and its makeFakeGatewayHarness(input) pattern, then locate the six NotionMdGateway fake implementations and the body-byte assertions. Use the live integration harness, OTEL span-shape tests, and notion-effect-client's createMockHttpClient as reference points. Done means the fakes share one harness, incidental body checks use semanticEquivalent, and demo/showcase.nmd has a golden-file fixpoint.