fix(cdp): expose content editing host accessibility values

[enginespot]

What changed

Fixes #730.

Accessibility value computation now exposes the current DOM text for content editing hosts.

The implementation follows contenteditable keyword and inheritance semantics, including true, the empty keyword, plaintext-only, false, invalid inherited values, and a nested host re-enabled inside a disabled region. Only the editing host receives the value, so editable descendants do not duplicate it. Native input, textarea, and select value handling remains unchanged.

Validation

• cargo nextest run -p obscura-cdp: 126 passed, 3 skipped
• Focused content editing host AX value regression test: passed

Rendering

Not applicable.

Performance

AX value computation walks ancestors only for non-native candidate elements while building the accessibility tree. The walk is bounded by DOM depth and introduces no persistent state or new dependency.

Checklist

• The change is focused and does not remove existing behavior without justification.
• Tests cover the failure or feature.
• Existing tests pass, including render and no-render configurations when affected.
• I checked for CPU, latency, and memory regressions.
• Public API or user-facing behavior changes are documented.