feat(browser): bound tunnel route resources

[Jinwoo-H]

	Files	Added	Deleted	Net
Test	1	$\color{#1a7f37}{\Huge{\mathbf{+}}}$​222	0	$\color{#1a7f37}{\Huge{\mathbf{+}}}$​222
Prod	7	$\color{#1a7f37}{\Huge{\mathbf{+}}}$​263	$\color{#cf222e}{\Huge{\mathbf{−}}}$​72	$\color{#1a7f37}{\Huge{\mathbf{+}}}$​191

Summary

• cap one route at 16 pending destination opens and 128 admitted opens per monotonic 10-second window
• share an 8 MiB application-retention ledger across both tunnel directions and cap unsettled destination writes at 256 chunks per stream
• release accounting exactly on settlement, connection, retirement, stale callbacks, and re-entrant transport close

Causal evidence

On parent 392d13b, the deterministic oracle admitted 17 pending opens, admitted a 129th open inside the same 10-second window, and retained more than 8 MiB across one execution-host route. The candidate makes those assertions green and adds independent client-to-destination, chunk-count, stale-callback, and re-entrant-close coverage.

Compatibility and activation

This is an inert STA-4150 layer stacked on #14493. It does not advertise a capability, register a production RPC method, add a shipped stream opcode, change existing server/offscreen placement, or depend on #14402. Old clients and server-hosted browsing remain unchanged.

Still blocked before activation: encrypted WebSocket/native buffer accounting; browser-host and process-wide ledgers; fair scheduling; reconnect and mixed-version journeys; SSH/WSL adapters; SOCKS trust isolation; Electron proxy/no-fallback proof; Linux and physical Windows evidence.

Validation

• 11 focused files / 73 tests
• full runtime RPC suite / 1,347 passed, 1 skipped
• typecheck
• native and type-aware code-quality audits
• reliability manifest and max-lines ratchet
• git diff check

Linear: https://linear.app/stably/issue/STA-4150/refactor-remote-browser-to-client-hosted-electron-webviews