Opt-in control-plane fallback for browser_gone 404s

Reworks the prior draft, which fell back to the control plane on any 5xx for any idempotent GET. That broad trigger meant a transient 502 from a healthy VM caused a retry of the dead VM and then a control-plane re-issue, adding latency for failures that should just be returned.

This replaces it with a tighter, opt-in, per-endpoint design that assumes the paired metro-api change kernel#2317 lands.

Depends on kernel#2317

When a routed request targets a deleted/gone browser, the VM proxy returns HTTP 404 with JSON body {"code":"browser_gone","message":"browser not found"}. There is no special response header — we key off the body code only. A transient/real upstream failure still returns 5xx; a live VM's own 404 has no browser_gone code.

New fallback semantics

The routing layer keeps a small registry of fallback-eligible routed paths, expressed as (subresource, suffix) and default-OFF for everything else.

We fall back to the control plane iff ALL hold:

1. the request was actually routed to the VM (allowlisted subresource + a cached route exists);
2. the HTTP method is GET;
3. the routed path is in the fallback-eligible registry;
4. the VM returned 404 and its JSON body code == "browser_gone".

We do NOT fall back on: success, transient 5xx (502/503/504), connection/network errors, other 4xx, or a 404 whose body code is not browser_gone. Those propagate unchanged.

On fallback we evict the cached route for the session (it's authoritatively gone), then re-issue the original request to the control plane exactly once (original CP URL, Authorization restored, jwt query param dropped). Never loops.

Prospective endpoint

Only the forthcoming pull endpoint GET /browsers/{id}/telemetry/events (("telemetry", "/events")) is pre-registered, clearly commented as PROSPECTIVE — the telemetry.events(...) method does not exist yet. This wires the opt-in so fallback works the moment that method ships. Adding any future eligible endpoint is a one-line registry edit.

Scoping

This PR is focused on the fallback. It does NOT modify the default routing subresource list (that belongs to the separate telemetry-default-routing PR). git diff origin/next does not touch the default-subresources constant; tests enable telemetry routing locally.

Verification

• tests/test_browser_routing.py: 33 passing. Covers eligible+GET+browser_gone -> CP fallback (Authorization restored, jwt dropped, route evicted, exactly one CP re-issue), browser_gone+CP-also-errors -> returned as-is no loop, non-eligible path / 502 / connection error / 200 / POST / plain-404 -> no fallback, and non-routed requests untouched, plus a unit test of the eligibility registry. Sync and async parity covered.
• ruff check clean on all changed files.
• Live scoping QA against prod (browser_gone not deployed yet; positive path proven by unit tests): (A) telemetry stream routed directly to the VM proxy host, not api.onkernel.com; (B) after deleting the browser, the stale telemetry-stream route returned the VM 502 to the caller with ZERO control-plane re-issues — correct, since /stream is not eligible and 502 is not browser_gone.

Files: src/kernel/_client.py, src/kernel/lib/browser_routing/routing.py, tests/test_browser_routing.py.