docs(release-notes): add #426, #427, #354 sprint cleanup entries

[xe-nvdk]

Doc-only follow-up after the sprint's three small PRs merged. Adds release-notes entries for:

• feat(metrics): arc_query_client_disconnects_total counter (closes #426) #466 (closes Add metric arc_query_client_disconnects_total for cancelled-mid-stream queries #426) — `arc_query_client_disconnects_total` Prometheus counter, placed under the existing PR fix(query): break streaming loop on Flush error so DuckDB result buffers free on client disconnect #424 entry as an observability follow-up.
• fix(test): Release buildArrowBatch builders + columns (closes #427) #465 (closes buildArrowBatch test helper leaks builder allocations; blocks CheckedAllocator usage in Arrow tests #427) — Arrow streaming tests now leak-detect with `CheckedAllocator`, placed as a sibling test-hygiene follow-up to PR fix(query): break streaming loop on Flush error so DuckDB result buffers free on client disconnect #424.
• perf(ingest): sub-slice indexing in splitOnDelimiter (closes #354) #467 (closes low(ingest): splitOnDelimiter byte-by-byte append #354) — Line Protocol parser sub-slice indexing in `splitOnDelimiter` with the bench table (ParseLine -41% ns / -52% allocs, ParseBatch -31% ns / -50% allocs); notes the declined dynamic-cap-8 Gemini suggestion that regressed the full path.

+35 lines, single file.

Test plan

• No code changes; doc-only.
• Gemini Code Assist review (just for proofreading; this is a doc PR).

🤖 Generated with Claude Code

[xe-nvdk]

@gemini-code-assist review please

[gemini-code-assist]

Code Review

This pull request updates the release notes for the 2026.06.1 release, documenting the addition of the arc_query_client_disconnects_total Prometheus counter, memory leak detection improvements in Arrow streaming tests, and performance optimizations in the Line Protocol parser's tokenizer. Feedback is provided to correct a minor technical inaccuracy in the Prometheus terminology used in the release notes, specifically clarifying that the different paths are "label values" rather than separate "labels."

[gemini-code-assist]

In Prometheus terminology, path is a single label, while arrow_ipc, arrow_json, and sql_json are label values (or dimension values). Describing them as "three labels" and "an unknown label" is technically inaccurate and can confuse operators or developers configuring dashboards/alerts.

Consider updating this line to refer to them as label values:

Three label values distinguish (a) the `/api/v1/query/arrow` IPC endpoint, (b) the `duckdb_arrow`-build path that `/api/v1/query` takes by default, and (c) the pure `database/sql` streaming JSON path. Wired at all 5 existing client-disconnect Warn-log sites in [internal/api/query_arrow.go](internal/api/query_arrow.go), [internal/api/query_arrow_json.go](internal/api/query_arrow_json.go), and [internal/api/query.go](internal/api/query.go). Guarded by the existing `isClientError()` predicate, so server-side stream failures stay in `IncQueryErrors` only — the new counter is strictly client-side noise. Typed `DisconnectPathArrowIPC` / `DisconnectPathArrowJSON` / `DisconnectPathSQLJSON` constants prevent typos at compile time; an unknown label value is silently dropped by the increment switch.

[gemini-code-assist]

Code Review

This pull request updates the release notes to document several recent changes, including the addition of the arc_query_client_disconnects_total Prometheus counter, leak detection in Arrow streaming tests using CheckedAllocator, and performance optimizations in the Line Protocol parser's tokenizer. A review comment points out a technical inaccuracy in the description of the Prometheus metric, noting that it has a single label with three possible values rather than "three labels."

[gemini-code-assist]

The Prometheus metric arc_query_client_disconnects_total has a single label named path with three possible values (arrow_ipc, arrow_json, and sql_json). Referring to them as 'Three labels' is technically inaccurate. It should be corrected to 'Three label values' or 'Three values for the path label' to ensure technical accuracy in the release notes.