M6-T2: window-protocol load harness (local-guarded, extensively tested)

[JaamesBond]

Builds the M6-T2 load harness for the hot path (window_open → window_beat×N → close_window) and validates it end-to-end against a local stack. Prod untouched — a hard guard refuses the prod ref / non-localhost.

What's here

• scripts/load/lib/hmac-chain.mjs — synthetic-client heartbeat HMAC chain, byte-matched to public.window_beat (msg="seq|prev|activity" keyed by challenge; prev chains to the previous beat's hmac, window_id for beat 1). Pure, node:crypto only.
• scripts/load/lib/metrics.mjs — pure latency percentiles (nearest-rank), throughput, successful-writes/s, (op,code) error taxonomy.
• scripts/load/harness.mjs — driver. assertSafeTarget() hard-refuses prod ref prmsonskzrubqsazmpwd or any non-localhost host. Seeds auth.users → publishers → devices (auth_user_id NOT NULL) via GoTrue admin + service-role inserts, mints device JWTs, runs concurrent virtual users at honest cadence (≥600ms beat spacing, full 5s dwell).
• test/load-harness.test.mjs — +7 hermetic cases; HMAC vectors locked to the server contract (drift → every real beat 400s).
• docs/ops/load-test-plan.md — methodology, safety guard, ceiling math (~15k writes/s @ ~50k devs), RL tuning, Cloudflare-DO migration trigger, gated run instructions.

Test evidence (local stack)

• Smoke (2 users, 14s): 30 ops, 0 errors; +6 windows, all 6 credited (credit path fires).
• Scale (100 concurrent users, 25s): 2500 ops, 0 errors; beats p99 37ms, opens p99 332ms (surfaces window_open's order by random() as the slowest op — a real finding). All 506 honest windows credited (HMAC chain + anti-batch + dwell correct under concurrency).
• Guards: prod-ref target refused; non-localhost refused without LOAD_ALLOW_NONLOCAL=1.
• Cleanup: cascade-deleted synthetic data → local DB restored to exact baseline (no residue).
• Suite: node --test test/*.test.mjs → 248/248 (+7).

Not shipped to npm. Running against anything shared still DEPENDS-ON M5 (prod is single-owner until the first charge reconciles).

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