fix(cua-driver): expose legacy macOS page fallback

[injaneity]

summary

• preserve the legacy page.get_text and page.query_dom text payload while attaching macOS read provenance in structuredContent
• label successful Apple Events reads as javascript, Electron reads as cdp, and direct or failure-driven AX reads as ax_fallback
• preserve the triggering JavaScript/CDP error, report bounded AX truncation, and retain both errors if AX fallback also fails
• document the compatibility-only fallback contract

fixes #2245

tests

• cargo test -p cua-driver-core --lib
• cargo test -p platform-macos tools::page::tests --lib
• cargo fmt --all -- --check
• git diff --check

cargo clippy -p cua-driver-core -p platform-macos --all-targets -- -D warnings remains blocked by pre-existing clippy errors in cursor-overlay, browser/engine.rs, browser/grant.rs, and recording code; none are in this change.