Make duckdb_arrow the default build path (drop the tag) so default go test exercises shipped code

[xe-nvdk]

Problem

Every shipping build of Arc sets -tags=duckdb_arrow (Makefile GOFLAGS, Dockerfile, .github/workflows/release-build.yml), so production runs the Arrow code path. But a bare go build ./... / go test ./... from a developer shell — or any tooling that doesn't pass the tag — silently compiles the non-Arrow path instead. The default developer/test experience exercises code that is not what ships.

This is a real foot-gun, not hypothetical. In PR #489 (SHOW-command RBAC gating), the executeQueryArrow handler (internal/api/query_arrow.go, //go:build duckdb_arrow) was missing the SHOW gate that the JSON and MsgPack endpoints had. A default go test ./... could not catch it because the file isn't compiled without the tag — it was found only by an internal review that explicitly built with -tags=duckdb_arrow. The Arrow path can rot undetected.

Note: make test does pass the tag (GOFLAGS=-v -tags=duckdb_arrow), so the Makefile path is correct. The gap is everything that runs go test directly (contributors, IDE test runners, ad-hoc CI steps, review agents).

Preferred fix: drop the build tag entirely

Per the prior decision (2026-05-12), the recommended approach is to remove //go:build duckdb_arrow and make Arrow unconditional, rather than teach every tool to pass the tag. Every production build already requires it; the opt-out path is effectively dead.

Files to change (verified present):

• internal/database/duckdb_arrow.go — strip //go:build duckdb_arrow
• internal/api/query_arrow.go — strip the tag
• internal/api/query_arrow_json.go — strip the tag
• internal/api/query_arrow_json_test.go — strip the tag
• internal/api/query_msgpack_test.go — strip the tag (also tag-gated)
• internal/api/query_arrow_stub.go (//go:build !duckdb_arrow, the no-op executeQueryArrow) — delete
• internal/database/ — check for a !duckdb_arrow stub counterpart to duckdb_arrow.go and delete it too

Then -tags=duckdb_arrow in Makefile / Dockerfile / release-build.yml is harmless to leave or can be dropped.

The blocker that deferred this (PR #434, arcx loader) merged on 2026-05-13, so it's unblocked.

Acceptance criteria

• go build ./cmd/... ./internal/... and go test ./... from a clean checkout compile and run the Arrow path with no tag required.
• No //go:build duckdb_arrow / //go:build !duckdb_arrow pairs remain in the tree (grep -rn duckdb_arrow --include='*.go' returns nothing in build constraints).
• go vet ./... clean; existing Arrow + MsgPack tests run by default.
• Confirm there is exactly one definition of executeQueryArrow after deleting the stub (no duplicate-symbol build error).

Scope / risk

Developer-experience + test-coverage hardening; no functional change to shipped behavior (production already builds with the tag). ~10–15 line diff plus two file deletions. Separate PR, not bundled with feature work, so the build-constraint change has its own git blame.