fix(cdp-client): type-only emitter cast for Page.loadEventFired race

[sergei-aronsen]

Summary

Main currently fails tsc with:

```
src/cdp-client.ts(1394,22): error TS2339: Property 'removeListener' does not exist on type 'Client'.
src/cdp-client.ts(1397,20): error TS2339: Property 'once' does not exist on type 'Client'.
```

Introduced by #18 (ebf055e). The CRI `Client` is an EventEmitter at runtime, but `@types/chrome-remote-interface` only declares `on(...)` — not `once`/`removeListener`. Build fails on every PR until this lands.

Fix

Locally-scoped EventEmitter-shaped interface + a single as unknown as ... cast. No runtime change.

Why it matters

CI is red on main, which means every open PR (#14, #15, #17, #20) is also red regardless of its own correctness. This unblocks all four.

Test plan

• npm run test:unit (55/55 pass locally)
• npx tsc --noEmit clean

🤖 Generated with Claude Code

[sergei-aronsen]

Heads-up: this PR unblocks four others that are mergeable but red on CI for the same tsc error:

• fix(upload): allowlist filesystem paths and validate tabId #14 — fix/upload-allowlist
• fix(cdp): harden Comet launch and reject non-http(s) navigation #15 — fix/cdp-launch-hardening
• feat(security): wrap page-derived response text in UNTRUSTED markers #17 — feat/untrusted-content-marker (Finding 1 + Finding 2 from your review addressed: nonce-based markers, coverage expanded to comet_ask timeout and comet_poll still-working paths)
• fix(misc): cryptographic taskId, hoisted fs import, graceful shutdown, awaited transport #20 — fix/misc-cleanups (disconnect timeout added per review)

Suggested merge order: this PR first, then the rebased four. Once this lands, hitting "Update branch" on each (or I can force-push an empty rebase to re-trigger CI) flips them green.

[RapierCraft]

Review: PR #22 — fix(cdp-client): type-only emitter cast for Page.loadEventFired race

Reviewed commit: dd778ee | Review type: Correctness
Verdict: APPROVE — merge ASAP, unblocks all other PRs

---

Analysis

• TypeScript compiles clean (tsc --noEmit exits 0) — this fixes the pre-existing build failure on main
• The CdpEmitter interface is correctly scoped inside the method block — minimal surface, no module-level pollution
• as unknown as CdpEmitter is the standard two-step cast for widening then narrowing — correct TypeScript pattern
• The interface exposes only once and removeListener — exactly what's needed, nothing more
• No runtime change — the CRI Client already has these methods at runtime, this just makes TypeScript aware

Priority

This should be merged first among all open PRs. Every other PR shows type errors from this same location, making CI red across the board. Merging this unblocks CI for PRs #14, #15, #17, #20, #23.

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Review complete. TypeScript: clean build. 0 findings.