Implements --dump-text[=<mode>] (innertext/accessibility/raw-dom) plus --idle and --max-wait tunables. Renderer-side text extraction via a content::WebContentsObserver in chromium/src/carbonyl/src/browser/dump_text_handler.{cc,h}, wired into headless_shell.cc::OnBrowserStart via patch 0027. Bypasses the terminal renderer for clean single-process invocation; chromium subprocesses inherit shell_mode=1 to prevent ANSI noise.

Verified end-to-end against runtime-a98928556d7c96c1: example.com body text extracted to stdout (130 bytes, exit 0), --dump-text=raw-dom emits full HTML (530 bytes), --idle and pipe semantics correct.

Known follow-ups (separate issues): accessibility mode pending fathyb#4, empty-error-page exit semantics, runtime-hash.sh excluding Rust sources, std::_Exit teardown warning.

Closes fathyb#88
Refs fathyb#4 fathyb#5

…athyb#93)

EmitAndExit now posts HeadlessBrowserImpl::ShutdownWithExitCode() to the UI thread instead of calling std::_Exit. Chromium runs its destructor chain in order; the WebContentsObserver hook fires and self-deletes the handler.

Verified against runtime-74bd7a3a33b7bf56: WebFrame LEAKED line gone from stderr; exit code 0 with correct text on stdout; example.com smoke test 130 bytes clean.

API: StartFor now takes (WebContents*, HeadlessBrowser*). Patch 0027 updated to pass the existing OnBrowserStart browser pointer through.

GN dep added: //headless:headless_browser (HeadlessBrowserImpl + ShutdownWithExitCode). Wider closure but :dump_text is only consumed by headless_shell, no cycle risk.

Closes fathyb#93
Refs fathyb#88

DumpTextHandler now observes DidFinishNavigation and exits 6 with empty stdout when the latest primary-frame navigation was a chromium error page or had a non-OK net::Error code. Previously, unreachable URLs exited 0 with empty stdout because chromium loads its internal error page as a real document and our handler extracted its empty body.

Verified against runtime-f6d15927b1241217:
  - https://example.com → exit 0, 130 bytes (regression check)
  - http://10.255.255.1:1/never → exit 6, empty stdout, logs net::ERR_UNSAFE_PORT
  - https://invalid.invalid/ → exit 6, empty stdout, logs net::ERR_NAME_NOT_RESOLVED

Known cosmetic: stderr log says net::net::ERR_* (double net:: prefix because ErrorToString already prefixes). Trivial fix; not worth a rebuild cycle.

HTTP error status codes (404, 500) are NOT treated as nav failures — those are real HTML served by the server.

Closes fathyb#91
Refs fathyb#88