[Feature] BanyanDB: BydbQL surface, integration fixtures, docs for tag aggregation

Part 4 of 5 of the tag aggregation and time bucketing design, shipped in apache/skywalking-banyandb#1360.

Design references: §5.4 BydbQL · §10 Compatibility (limit) · §11 Testing strategy · §12 Delivery order (stages 7–8)

Depends on: #14089 (time-bucket grouping) and #14090 (COUNT_DISTINCT) — this issue makes both reachable from BydbQL and proves them end to end together.

Scope

BydbQL

• COUNT(DISTINCT col) — DISTINCT valid inside COUNT only; SUM(DISTINCT col) is a parse error.
• Aggregate column resolves against tags as well as fields (convertAggregation currently hard-errors outside allFields).
• TIME_BUCKET(<duration>) pseudo-column, valid only in GROUP BY, at most once, argument optional (empty → the measure's own interval); not selectable in the projection.
• bydbctl TUI planner constant so the plan renders the new function/pseudo-column.

Integration

• The load-bearing multi-bucket cardinality fixture (one user × 3 api-keys / 3 hour buckets, a second user × 2), asserting the scalar case and the correct order for the ranked+paged case — run in both test/integration/distributed/query/vectorized_test.go (liaison + 2 data nodes) and standalone.
• The same fixture reused at a second bucket width, to catch a bucket key that's computed but ignored.
• The decomposition oracle: a bucketed query over a window must equal the union of per-bucket queries over the same window, row-for-row on (bucket, tags…) — each per-bucket query is independently trustworthy, so this is a real oracle rather than a golden file.
• data.go's golden comparison currently ignores timestamps; these new cases need it included, or every bucket silently collapses to zero and the suite still passes.
• The generated feature matrix (test/cases/measure/cmd/generate/layer3_features.go, ql_renderer.go) gains the sixth aggregate function — it's hard-coded today and won't be caught by the exhaustive linter (the switches have default branches).

Docs

• docs/interacting/bydbql.md: the new grammar, and prominently, the limit-with-bucketing note (default 100 truncates a bucketed series — this must be visible in the docs, not just the API reference).
• docs/api-reference.md regenerated, never hand-edited.

Out of scope

• The optional local-scan performance follow-up (run-folding, metadata-only scan) — separate issue.

Test / Definition of done

• Parser: COUNT(DISTINCT x) parses; SUM(DISTINCT x) errors; TIME_BUCKET()/TIME_BUCKET('5m') both parse; a second TIME_BUCKET in one GROUP BY errors; TIME_BUCKET in SELECT errors.
• Binder/transformer protocmp equivalence tests against the equivalent hand-built QueryRequest.
• bydbctl planner test rendering a query using both features.
• The integration fixtures above green in standalone and distributed CI.
• Doc diff reviewed specifically for the limit callout.