perf(pglite): reduce per-row allocation in parseResults

[huntiezz]

Summary

Optimizes the hottest JS-side path in PGlite — converting PostgreSQL wire-protocol DataRow messages into JavaScript objects/arrays in parseResults().

Problem: Every row previously allocated intermediate structures:

• Object mode (default): Object.fromEntries(msg.fields.map(...)) — builds a [key, value][] array per row, then iterates it again
• Array mode: msg.fields.map(...) — callback-based allocation per row

Field names and type OIDs were also re-read from currentResultSet.fields[i] on every row even though they're constant for the entire result set.

Fix:

1. Hoist fieldNames[] and fieldTypes[] once per result set on rowDescription
2. Build object-mode rows with a tight indexed for loop (no Object.fromEntries)
3. Preallocate array-mode rows with new Array(n) and fill by index
4. Reset hoisted metadata on commandComplete

This is a behavior-preserving micro-optimization — a natural follow-up to #495, focused on large result-set workloads (#630). Does not overlap with maintainer drafts #973 (parse redesign) or #903 (streaming parse).

Test plan

• New unit tests in packages/pglite/tests/parse.test.ts (9 cases: object/array modes, multi-result sets, INSERT/UPDATE/DELETE counts, custom parsers, blob attachment, describe helper)
• pnpm vitest tests/parse.test.ts — 9 passed
• pnpm test:basic — 278 passed (57 files)
• pnpm stylecheck && pnpm typecheck — clean
• Changeset added for @electric-sql/pglite patch

[AntonOfTheWoods]

@huntiezz you mention "optimize" - do you have any benchmarks showing what the actual gains might be?