*: Phase 2.B — vendor pqarrow/builder + arrowutils into pkg/query/internal

[thorfour]

Summary

Phase 2.B of the FrostDB removal. Stacks on #6354 (Phase 2.A).

Drops the pqarrow imports from the query path by vendoring the two helper packages.

Why vendor instead of replacing with arrow-go stdlib builders

The Opt* builders (OptInt64Builder, OptInt32Builder, OptBooleanBuilder) expose random-access mutation methods — Set(idx, v), Add(idx, delta), Value(idx), AppendData([]int64) — that the flamegraph and table query algorithms rely on for backfilling cumulative counts and child indices. arrow-go's array.*Builder is append-only, so swapping would require rewriting both algorithms. Vendoring is mechanical, low-risk, and unblocks Phase 4 (drop frostdb from go.mod).

What's vendored

• pkg/query/internal/builder/ — listbuilder.go, optbuilders.go, recordbuilder.go, utils.go from frostdb/pqarrow/builder. Plus optbuilders_test.go (passes against the vendored copy). The AppendParquetValues methods on the Opt builders and the parquet-go import are stripped — parca uses these builders only on the query side and never to convert parquet values, so they're dead code.
• pkg/query/internal/arrowutils/ — groupranges.go, merge.go, nullarray.go, schema.go, sort.go, utils.go from frostdb/pqarrow/arrowutils. Plus sort_test.go. merge_test.go and schema_test.go are skipped — they depend on frostdb/internal/records, which is not importable from outside the frostdb module.

Each vendored package gets a doc.go explaining provenance.

Consumers updated

• pkg/query/{flamegraph_arrow,table}.go: import path swap from frostdb/pqarrow/builder to internal/builder.
• pkg/query/columnquery.go: import path swap from frostdb/pqarrow/arrowutils to internal/arrowutils.
• pkg/parcacol/querier.go: still on the upstream arrowutils because pkg/parcacol can't import pkg/query/internal/arrowutils. That file goes away in Phase 3.

go.mod: parquet-go is demoted from direct to indirect (still pulled in by frostdb itself; goes away in Phase 4).

Test plan

• go build ./...
• go vet ./...
• go test -short ./...
• Vendored builder tests pass (go test ./pkg/query/internal/builder/...)
• Vendored arrowutils sort tests pass (go test ./pkg/query/internal/arrowutils/...)
• CI green
• Manual smoke: open a flamegraph against a ClickHouse-backed Parca

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_{Expected differences? Click here. Last updated for commit 299a0c0 *: Vendor pqarrow/builder + pqarrow/arrowutils into pkg/query/internal. This comment will update as new commits are pushed.}