fix(server): guard #send against closed sockets — no uncaught throw on abrupt sub

[grrowl]

Fixes #40

Problem

#send (src/server/mixin.ts) called ws.send(encoded) unconditionally — no readyState check, no try/catch. When a client sends sub and the socket transitions to CLOSING/CLOSED before the snapshot finishes streaming (normal churn: dispose, navigate-away, StrictMode teardown, forced reconnect drop), ws.send() throws Can't call WebSocket send() after close(), surfacing as an uncaught exception.

Fix

Guard #send: skip when readyState !== WebSocket.OPEN, and wrap ws.send in try/catch to cover the OPEN→closed race the pre-check can't. A post-close send is treated as a benign no-op — the socket's webSocketClose already tears down its subs (#dropSocketSubs), so no additional cleanup is needed (stated in a code comment).

• Outbound-only; no state impact, no behavior change for OPEN sockets.
• The Broadcaster egress path is covered too — it routes every send through the same #send closure (new Broadcaster((ws, frame) => this.#send(ws, frame), …)), the single raw-send chokepoint.

Test

New test in tests/ws-lifecycle.test.ts pins the abrupt-close-mid-snapshot shape against a real hibernatable DO socket: open the socket, close the server-side socket, then hand it the sub frame so the terminal snap-end #send lands post-close. It asserts webSocketMessage resolves rather than rejecting.

Verified to fail on unpatched main — before the fix it rejected with the exact issue symptom:

TypeError: Can't call WebSocket send() after close().
  at SyncTestDO.#send src/server/mixin.ts:1096
  at SyncTestDO.#handleSub src/server/mixin.ts:1048  (snap-end)

After the fix: passes.

Validation

• npm test — full suite green (265 passed).
• npm run typecheck — clean.

This is a bug fix within existing decisions (outbound-only, ADR-0002/0006 unaffected) — no ADR. CHANGELOG entry added under [Unreleased] → Fixed.

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