Use an options object for searchByIndex and the custom-index search() contract

`searchByIndex` in `resources/search.ts` has grown to eight positional parameters, the last of
which — `minResults` — was added in [#2125](https://github.com/HarperFast/harper/pull/2125) so a
query's `offset + limit` could reach an approximate index and stop it from silently truncating the
result set.

```ts
export function searchByIndex(
searchCondition: DirectCondition,
transaction: any,
reverse: boolean,
Table: any,
allowFullScan?: boolean,
filtered?: any,
context?: any,
minResults?: number
): AsyncIterable;
```

Five of those eight are optional, so most call sites pass a run of `undefined`s or stop early and
inherit defaults they never meant to choose. `resources/Table.ts` and the recursive relationship
calls inside `search.ts` pass nothing past `filtered`, which is harmless today only because none of
them lead with a vector sort.

The real cost is on the custom-index contract. `HierarchicalNavigableSmallWorld.search()` is the
in-tree implementation of an interface an external index can also implement, and its signature grew
the same way — `(searchCondition, context, filter?, minResults?)`. Every future capability that has
to reach an approximate index (a cursor for stable paging, a deadline, a recall target) is another
positional argument, and adding one is a breaking change for any index implemented outside this
repo. The contract is still unreleased, so reshaping it is cheap now and expensive later.

## Suggested shape

Collapse the optional tail into an options object on both sides:

```ts
searchByIndex(searchCondition, transaction, reverse, Table, {
allowFullScan,
filtered,
context,
minResults,
});

customIndex.search(searchCondition, context, { filter, minResults });
```

The four leading parameters are required at every call site and read fine positionally; it is the
optional tail that should become named. Whether the custom-index `search()` keeps `context`
positional is a judgment call — it is passed by every caller, so either works.

Worth deciding as part of this: whether the options object is also where a future paging cursor
lands, since #2125 documented `offset`/`limit` over an approximate index as best-effort rather than
a stable partition (see the "An approximate index returns at most `ef` rows" section in `DESIGN.md`).

## Scope

- `resources/search.ts` — `searchByIndex` signature, the two recursive call sites, and the
`customIndex.search()` call.
- `resources/Table.ts:4960` — call site.
- `resources/indexes/HierarchicalNavigableSmallWorld.ts` — `search()` signature.
- Custom-index documentation, if the contract is written down anywhere outside the code.

Mechanical; no behaviour change intended. Raised by @maurice-harper in review of #2125 and recorded
as item 6 of that PR's "For the human reviewer" section, where the positional argument was kept to
avoid reshaping the contract inside a correctness fix.

Start in resources/search.ts at searchByIndex, its recursive calls, and the customIndex.search() call; then inspect resources/Table.ts:4960 and the search() signature in resources/indexes/HierarchicalNavigableSmallWorld.ts. Check the approximate-index guidance in DESIGN.md and any custom-index documentation. Done means the optional arguments use named options at all listed call sites and the search behavior remains unchanged.

Written by the indexing model from the issue text.