feat(vector): pre-filtered ANN via similar_to(..., filter: <var>)

[shaunpatterson]

Pre-filtered ANN: similar_to(..., filter: <var>)

Adds a filter: option to the similar_to vector-search function that scopes the
ANN search to the uid set of a DQL variable. The membership predicate is applied
during the HNSW graph traversal (not as a post-filter), so a scoped search
explores enough of the graph to return k in-scope neighbors instead of
post-filtering a fixed k and under-returning.

DQL

{
  allowed as var(func: eq(group, "A"))
  result(func: similar_to(emb, 5, "[0.0, 0.0]", filter: allowed)) {
    uid
    group
  }
}

The filter var is registered as a NeedsVar uid dependency, so the query
scheduler resolves it before the search runs; its resolved uids travel to the
worker via the task UidList and become the search allow-set.

Design notes

• In-traversal filtering. uidMembershipFilter builds a SearchFilter from a
  sorted copy of the allow-set + binary search (O(log n)); it copies before sorting
  so it never mutates the shared var list, and skips the sort when the input is
  already ascending (the common case — uid vars reach the worker sorted). A compact
  8-bytes/uid slice avoids the GC pressure of a multi-million-entry map for
  whole-tenant scopes.
• Wider candidate budget. A filtered search routes through the options path even
  without an ef: override, so it uses the max(k, efSearch) bottom-layer budget
  rather than the narrow k-only budget — needed to traverse past out-of-scope
  nodes and still return k in-scope results.
• Empty scope rejects everything (no silent fallback to a global search): a
  marker arg distinguishes "filter requested, scope empty" from "no filter".

Tests

• Unit (dql): filter option parsing, ef: combination, undefined-var error,
  duplicate-filter error, non-name value error, NeedsVar propagation.
• Integration (query/vector): scopes to filter var (returns k in-scope despite
  closer out-of-scope vectors), works without an ef: override, empty scope returns
  nothing, and a superset scope equals the unfiltered top-k.

All parser unit tests and the 4 integration tests pass.

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