Runtime guard on HierarchicalNavigableSmallWorld.search() options argument (follow-up to #2165)

Follow-up from #2165 / PR #2187.

PR #2187 moved `customIndex.search()` to `search(condition, context, { filter, minResults })` and added a runtime `TypeError` guard to `searchByIndex` so a stale positional 5th argument fails loud instead of silently permitting a full scan. The guard is asymmetric: `HierarchicalNavigableSmallWorld.search()` still destructures a positional `filter` function as `{ filter: undefined }` and drops it silently.

This is not theoretical. While merging main into #2187, `unitTests/resources/vectorIndexPlane.test.js:81` (added on main after the branch point) called `customIndex().search(cond, ctx, filter)` positionally, auto-merged clean, and two plane tests failed until it was converted by hand.

Scope:
- `typeof options !== 'object' || options === null` guard at the top of `search()` in `resources/indexes/HierarchicalNavigableSmallWorld.ts`, mirroring the `searchByIndex` guard.
- One test asserting the positional form throws.

Related, larger question raised by @kriszyp on #2187 and deliberately left out of that PR: if the custom-index contract is to become an extension point, it needs a typed `CustomIndex` interface and a registration API instead of the hard-coded `CUSTOM_INDEXES` literal in `resources/indexes/customIndexes.ts`. That is a separate design decision, not a shim.

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Start in resources/indexes/HierarchicalNavigableSmallWorld.ts at search(), and compare its argument handling with the searchByIndex guard. Run unitTests/resources/vectorIndexPlane.test.js, then add the focused assertion for the positional form. Done means the positional options call throws a TypeError rather than being silently accepted.

Written by the indexing model from the issue text.