Merge latest upstream and preserve fork hardening

[dudetru25]

What changed

• Merges the latest four commits from tnayuki/Unity-Unterm:main into the hardened Rank Up Games fork without discarding the fork's existing commits.
• Preserves the assembly-backed external-editor identity required when Unterm is vendored beneath another Unity package, while retaining upstream's package-update selection recovery.
• Combines upstream Markdown/debugger preferences with the fork's Unity MCP controls.
• Regenerates the locked Rust dependency graph and third-party notices for the new debugger stack.
• Builds, packages, checksums, and attests the standalone debugger executable on macOS and Windows; fixes the Windows .exe lookup.
• Updates GitHub Actions to pinned Node 24-compatible releases and pins the macOS runner to macos-15 to remove the reported build annotations.

Why

The fork was four commits behind upstream and GitHub could not synchronize it automatically because both sides changed the external-editor and settings integration. A direct upstream merge also exposed two fork-CI lint failures, stale legal notices, and a packaging gap where the new debugger executable was never shipped to Unity consumers.

Validation

• cargo test -p unterm --lib --locked — 55 passed, 1 ignored
• Fork CI Clippy command with --bins on Rust 1.94.1 — passed
• cargo audit --file native/Cargo.lock — passed (four existing policy-allowed unmaintained warnings)
• cargo deny --manifest-path native/Cargo.toml --config native/deny.toml check advisories bans licenses sources — passed
• npx --no-install github-actionlint .github/workflows/split-upm.yml — passed
• bash -n native/build-macos.sh — passed
• git diff --cached --check — passed before commit
• Pinned action SHA/version sweep, Unity meta GUID uniqueness, and generated notice checks — passed

GitHub Actions remains the authoritative cross-platform build proof for the universal macOS executable and Windows executable.