perf: pack logical-type name lists (allocNames blob)

[adubovikov]

Summary

• Replace per-label C.CString in allocNames with one pointer array + one contiguous NUL-terminated blob (duckdb_malloc), and freeNameList to release both.
• Remove duckdb_go_bindings_free_names from include/duckdb_go_bindings.h (callers free the blob/array only).
• Update CreateUnionType, CreateStructType, CreateEnumType, hybrid AppenderCreateQuery (column names only; query/table still use CString until a follow-up PR), and Arrow NewArrowSchema.
• Add TestCreateEnumType_manyPackedNames and BenchmarkCreateEnumType_64Names.

This is PR 1 of 4 splitting the work from PR #82 as requested for easier review. After this merges, the next branches to rebase onto upstream/main and open as separate PRs are:

• adubovikov:perf/upstream-pr/2-hot-bind-paths — bind/blob/varchar/vector hot paths without pool
• adubovikov:perf/upstream-pr/3-nul-pool-core — withNULString pool + Open/Query/Prepare paths
• adubovikov:perf/upstream-pr/4-nul-pool-sweep — remaining CString routes through the pool

Optional harness (after PR 1–4 land or separately): adubovikov:perf/upstream-pr/5-cpu-bench-harness (cpu_bench/).

Test plan

• CGO_ENABLED=1 go test ./...
• CGO_ENABLED=1 go test -tags duckdb_arrow ./...

[adubovikov]

Draft follow-ups in the same split (rebase each onto main after prior PRs merge): #84 #85 #86 - optional harness: #87.

[mlafeldt]

Thanks, arena-style allocation is the right call.

Just some minor cleanups left before we can merge this.

[adubovikov]

Pushed 8794e02 addressing review: removed unused duckdb_go_bindings_set_name, dropped redundant if len(columnNames) > 0 before allocNames/freeNameList, renamed L → slen, use uintptr(charSize) for name pointer stride, removed unused benchMustOpen from this PR’s bench file only. go test ./... and -tags duckdb_arrow pass.

[adubovikov]

Follow-up on review feedback

Thanks for the review - updates are in 8794e02 on perf/upstream-pr/1-alloc-packed-names.

Topic	Change
C helper	Removed duckdb_go_bindings_set_name; names are filled only from Go via memcpy into the packed blob.
AppenderCreateQuery	Always namesAlloc := allocNames(columnNames) + defer freeNameList(namesAlloc) - empty slice is handled inside allocNames, and freeNameList is nil-safe.
Naming / style	Loop length variable renamed from L to slen (avoids uppercase-looking short names).
Pointer stride	Name slot indexing uses uintptr(charSize) instead of unsafe.Sizeof(uintptr(0)).
Benchmarks	Dropped unused benchMustOpen from this PR's bench file only - later PRs in the split still add shared helpers where needed.

Checks: CGO_ENABLED=1 go test ./... and go test -tags duckdb_arrow ./... - green.

Let me know if anything else should be tweaked before merge.