chore(lync): split engines floor to true core floor + emit inspectable loss-free trial artifact

[deepfates]

EXHALE housekeeping — two charter-remnant debts. Does NOT touch the presence lane (PR #8).

(1) Split the engines floor — dee-oc37 / charter PR#5 item 4

package.json declared a monolithic engines.node ">=22" for the whole package, forcing every consumer onto Node 22 solely for the WebSocket helper.

• Audit: the browser-safe core (parse / store / views / looms / indexes / client) uses no node: module and no WebSocket — its highest runtime requirement is the Web Crypto global (globalThis.crypto in uuid.ts, crypto.randomUUID() in memory.ts and indexes/memory.ts), unflagged since Node 19. That is the true floor.
• Relaxed engines.node to >=19.
• Pinned the strict requirement where it applies, not on the whole package: the ws sync transport (createWebSocketTransport) and lync sync reach for the built-in WebSocket global, unflagged only on Node >=21 (per synced-store.ts:17). Documented per-surface in the README (Library / Live sync / Command), on the createWebSocketTransport docblock, and in cli/sync.ts — which also had an imprecise "global since Node 22" comment, now corrected to 21.
• Consumers on Node <21 can still use the transport by passing options.WebSocketImpl.

(2) Emit an inspectable artifact from the loss-free trial

The milestone-6 loss-free trial (dee-i1wc) proved durable sync but left only a green vitest run. It now writes a person-readable docs/trials/loss-free-trial.json — parity with golarion's dee-3fxq trial artifact — recording:

• every appended id, per client;
• where each landed — each client's store AND the relay's on-disk .lync;
• every surfaced failure (persist-failed, generation-changed), deduped per client;
• the three legs (disconnect, storage failure, restart) and what each surfaced.

The test asserts the artifact describes an actually loss-free, nothing-hidden run before it is trusted. 'Done' is no longer quietly 'tests pass'.

Proof

• pnpm verify green: 111 tests / 18 files pass, typecheck clean, README examples pass.
• Artifact generated at docs/trials/loss-free-trial.json (committed snapshot).

Do NOT merge / publish — coordinator's key.