feat(cua-driver): add internal action truth records

[f-trycua]

Summary

• add a rich, non-serialized ActionExecutionRecord at the canonical dispatch seam
• normalize the existing macOS, Windows, Linux, and browser action payloads into five stable route families without changing MCP bytes
• retain requested and actual delivery separately and downgrade legacy confirmations without trusted readback
• add internal action truth to opt-in trajectory artifacts while keeping request targets, coordinates, scope, and debug-only evidence out of the stable projection

This is intentionally stacked on #2705 and has been replayed onto its certified head e52ff35fdc2756194659d442ce03841dbfd51162. It should be rebased to main after #2705 merges. The public ActionResult contract and SDK accessors remain a separate atomic breaking PR.

Compatibility

• ToolResult.action_record is #[serde(skip)]
• legacy structuredContent is returned unchanged
• contract manifest and generated SDK bindings remain unchanged
• recording action.json gains only an additive action_truth field

Verification

Freshly rerun on rebased head 68b6e7b776ffec5fd8302ff77b585437f28a6ac2:

• cargo test -p cua-driver-core --lib --locked (448 passed)
• cargo test -p platform-macos --lib --locked (259 passed)
• cargo check -p cua-driver --locked
• cargo test -p cua-driver-contract --lib --locked (21 passed)
• cargo run --locked -p cua-driver-contract --bin cua-contract-gen -- all --check
• cargo fmt --all -- --check
• git diff --check

Architecture was reviewed with Claude Code Fable against the exact #2705 head. Its key recommendations incorporated here are the core/contract boundary, constructive no-echo projection, browser-action inclusion in the breaking bump, and removal of the window-scope validation gap in the follow-up.

[f-trycua]

Fresh exact-head CI completed for 68b6e7b776ffec5fd8302ff77b585437f28a6ac2: all 28 checks completed with 26 successes, 2 intentional skips, and 0 failures. This includes Linux and Windows Rust builds, portable contract parity on Linux/macOS/Windows, generated bindings, docs, release metadata, packaging, and the full Nix matrix. The PR remains stacked behind #2705 and will be rebased to main after that prerequisite merges.