fix(engine): keep an explicit null consolidated output instead of raw

Fixes #104.

What changes

At settle, the engine picked the consolidated output with nullish coalescing:

output = consolidated.output ?? raw; // engine/engine.ts:918

Consolidated.output is zJson.optional() — null is valid JSON and a valid consolidated result, and the hook contract (shared/core/schema/hooks/usage-consolidate.ts) says "absent = unchanged". Nullish coalescing treats explicit null as absent, so a hook returning { credits: {...}, output: null } had its result overwritten by the raw response — re-exposing exactly the vendor billing fields the hook stripped.

The settle path now falls back to raw only when consolidated.output is undefined:

output = consolidated.output === undefined ? raw : consolidated.output;

Verification

No deno on this host, so:

• HEAD-vs-fix semantics of the exact expression, Node 26: with consolidated = { credits: {}, output: null } and raw = { results: [...], vendorTotal: 0 }, HEAD produces raw, the fix produces null; with output omitted (undefined), both produce raw — the "absent = unchanged" case is preserved.
• Grep over engine/engine.ts: line 918 was the only ?? raw / consolidated-output fallback; the new fromResponse and doc.output.schema steps downstream are untouched and both already accept null (free-form outputs; zJson includes null).
• Added a Deno.test regression in engine/engine.test.ts next to the empty-claim test, modeled on the same usageUnit harness: explicit { credits: {}, output: null } settles output === null, and an omitted output still passes the raw payload through. I could not run deno test locally; CI is the authoritative run.

Summary by CodeRabbit

• Bug Fixes
  • Explicit null outputs from consolidation are now preserved, while omitted outputs continue to use the raw payload.
  • Usage is still settled when the consolidation claim is empty.