findings(E-1,E-2,E-3,E-6): ngspice service generations + credit-bounded event transport

Adapted from codex/asyncify-execution-owner-core 3753320. Service side mirrors
the occ-service shape (E-1 watchdogs, E-2 fail-all + boot-death fix — onerror
now rejects the in-flight boot waiter instead of stranding it, E-3
onmessageerror terminal, Blob URL revoked, per-generation evtQueue cleared on
retirement).

E-6 transport bounds (worker hunks re-applied inside the emscripten-6
em-pthread else-branch — the codex file predates that split, so this is a
re-application, not a cherry-pick):
- batch cut at 512 lines / 1 MiB exact JSON-UTF-8 bytes, measured before a
  line is retained; a single line > 1 MiB flushes the accepted prefix then
  stops the event stream terminally (never retained);
- posting gated by a 64-frame / 8 MiB unacked credit window; each frame
  carries { eventSequence, eventBytes } and is released only by an exact
  { sequence, bytes } ack; any mismatched ack is terminal;
- the service mirrors the same 64-frame / 8 MiB bound on its pre-handler
  queue, acks after handing a frame to __ngspiceOnEvent, and retires the
  generation on invalid credit; { fatal } frames retire the worker.

Tests: ngspice-service.test.ts (11, ported) — watchdogs, crash/bootError/
decode-fault settlement + recovery, out-of-order ids, sync postMessage throw,
stale-generation event drops, fatal-frame retirement. tests/tools/
ngspice-worker-batch-unit.ts (node:vm over the production worker source;
`npm run ngspice:worker-batch`) — bounded ordered chunks, byte-pressure
flush, 100k-chunk credit storm, over-limit line, exact ack lease. e2e harness
twin updated to speak the ack protocol (adds __ngspiceServiceTestHooks).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/package.json

@@ -29,7 +29,8 @@
     "3d:check:parity": "tsx tools/screenshots/compare-dirs.ts --old 3d-regression/baseline --new 3d-regression/output/webgl --out 3d-regression/output/diff/parity --floors 3d-regression/floors.json --level webgl-vs-native --label 3d-parity",
     "3d:review": "tsx tools/screenshots/compare-dirs.ts --old 3d-regression/baseline --new 3d-regression/output/webgl --out 3d-regression/output/diff/parity-review --floors 3d-regression/floors.json --level webgl-vs-native --label 3d-parity --artifacts always",
     "3d:test:webgl": "playwright test --project=wx-chromium e2e/3d-webgl.spec.ts",
-    "tools:contract": "tsx tools/cli-contract.ts"
+    "tools:contract": "tsx tools/cli-contract.ts",
+    "ngspice:worker-batch": "tsx tools/ngspice-worker-batch-unit.ts"
   },
   "devDependencies": {
     "@playwright/test": "^1.62.1",