perf: stored procedure for batch commands (appendBatch single round-trip)

[PaulGrimshaw]

Problem

append(AppendCommand[]) with per-command conditions currently uses appendBatch — a multi-round-trip path:

1. BEGIN
2. Advisory lock acquisition (1 query)
3. getHighWaterMark (1 query)
4. COPY events to main table
5. CREATE TEMP TABLE + COPY conditions
6. JOIN-based condition check
7. currval
8. pg_notify
9. COMMIT

~10 round-trips per call. Benchmarked at 2,600 ev/s for 500 commands × 1 event each (T16).

Compare: single-command appendViaFunction does everything in 1 round-trip via stored procedure — benchmarked at 22,000 ev/s (T1) for single-event conditional appends.

Proposed solution

A stored procedure that accepts arrays of commands and does the full loop server-side:

1. Acquire advisory locks (already done in SP for single commands)
2. INSERT all events via unnest/generate_subscripts
3. Per-command condition check against pre-insert snapshot (highWaterMark pattern)
4. RAISE on first violation with command index
5. pg_notify + return last position

Single round-trip. Expected gain: 5-8x for batch commands with per-event conditions.

Benchmark baseline (from event-store-bench T16)

Cmds/append	Current ev/s	p50
1	811	6ms
10	1,619	45ms
50	2,000	271ms
100	2,160	524ms
500	2,600	2,148ms

Target: match or approach T1's 22K ev/s at 1 cmd, and scale with batch size.

Context

• Discovered via T14/T15/T16 scenarios in packages/event-store-bench
• COPY_THRESHOLD raised to 10K in feat: event-store-bench + raise COPY_THRESHOLD to 10K #78 based on T15 crossover analysis
• The stored procedure path already exists for single commands — this extends it to handle multiple commands