AGC Unity Attach helper

Includes portions of DotCraft Unity 0.4.3 by DotHarness.
Source: https://github.com/DotHarness/dotcraft-unity
Commit: a65f091c162ebcfde5ec9b399d188fcc57261947
License: Apache License 2.0; see vendor/LICENSE.
Original file hashes and source paths: vendor/upstream.json.
Hashes are SHA-256 of raw Git blob bytes at the pinned commit, before
checkout line-ending conversion; they do not include local modifications.

Local modifications (2026-09-18):
- Separate Attach build from DotCraft.Harness and Unity package metadata.
- Add execution dispatch observation and deadline checks around compilation.
- Propagate cancellation through discovery/handshake/compilation preparation.
- Preserve transport uncertainty separately from confirmed domain loss.
- Add local selection invalidation for failed or changed project connections.
- Expose only the controlled AGC NDJSON host protocol, identity validation,
  message limits, deadlines and irreversible unknown-outcome execution latch.

No DotCraft.Harness, MCP server, chat UI or Unity UPM package is included.