…snapshot

--dump-text=accessibility previously fell back to document.body.innerText
with a LOG(WARNING) because the backing accessibility FFI had not landed.
That FFI shipped (fathyb#4): AccessibilityHandler takes a browser-process AX
snapshot and serializes it to JSON (carbonyl_get_accessibility_tree /
GetTreeJSON), and chromium patch 0028 installs the handler unconditionally
in OnBrowserStart with ui::kAXModeWebContentsOnly forced on.

OnIdleElapsed now intercepts kAccessibility before the JS-eval path and
emits AccessibilityHandler::GetTreeJSON() directly: synchronous, on the UI
thread, never null, heap string released via carbonyl_free_string(), with
the {"error":"no_tree"} sentinel on failure. ScriptForMode no longer routes
kAccessibility (kept as an unreachable switch arm for exhaustiveness). Adds
the :dump_text -> :accessibility GN edge; both already depend on
//content/public/browser and :accessibility does not depend on :dump_text,
so no cycle is introduced.

Also drops the doubled "net::" literal in the navigation-failure log line
(net::ErrorToString already prepends it).

Verification: this is Chromium-side C++ that the push-time CI (cargo check +
clippy + lib tests) does not compile; it requires a runtime build
(build-runtime.yml on titan). Validate with
`carbonyl --dump-text=accessibility <url>` emitting the role/name/value
JSON tree.

Refs fathyb#90, fathyb#97

…apshot

Lands fathyb#90 (browser-process AX snapshot for --dump-text=accessibility) and
fathyb#97 (drop doubled net:: prefix in the nav-failure log). Reviewed by
inspection; Chromium-side C++ to be validated by the next build-runtime.yml
run on titan (headless x86_64).

Closes fathyb#90
Closes fathyb#97

- RELEASE-v0.2.0-alpha.8.md: notes for the delta since alpha.7 — real
  accessibility-tree output for --dump-text=accessibility (fathyb#90), the net::
  log-prefix fix (fathyb#97), and automated macOS release-asset staging (fathyb#113/fathyb#117).
  Chromium patch stack unchanged at 30.
- package.json: 0.2.0-alpha.7 -> 0.2.0-alpha.8 (Cargo.toml version left as-is
  per convention; it is a runtime-hash input).
- changelog.md: regenerated via git-cliff — adds the alpha.8 section and
  backfills the previously-missing alpha.7 section.
- docs/ci-runner-mutsu.md: note that release.yml now auto-stages/mirrors the
  macOS asset; only the build trigger remains manual.

Runtime hash for this release: 283ca65ffeeaa2dc. Cutting the v0.2.0-alpha.8 tag
requires runtimes built for that hash (titan headless+x11; mutsu macOS, or cut
with include_macos=false for a Linux-only release).