Improve editor performance and MCP controls

[dudetru25]

Summary

• Restores the terminal tab icon after editor reloads and live title changes, while preserving the Claude Code and code-editor icons.
• Removes Unity-side Claude Code downloading/installing/updating; the optional panel now discovers an existing user-managed claude executable from UNTERM_CLAUDE_PATH, PATH, common locations, or a legacy install.
• Caches zero-copy texture wrappers and theme reflection, avoids redundant native resize work, and throttles idle GPU, diff, and file polling.
• Keeps Unity MCP dormant until enabled for the current project and adds Prompt, Allow Mutating, and Allow Dangerous access policies.
• Keeps arbitrary C# behind a separate full-machine-access opt-in; unknown tools and unattended actions outside policy remain fail-closed.
• Preserves an audited future Linux debugger dependency path using target-scoped X11 features. Wayland remains excluded because its current transitive quick-xml graph is covered by active RustSec advisories.

Platform scope

• The current embedded source-pane renderer and produced native debugger binaries remain macOS/Windows only.
• CI verifies the Linux/X11 dependency graph and rejects accidental Wayland activation; this records and protects Linux debugging intent without falsely claiming a shippable Linux renderer.

Security and supply chain

• Project policy is stored in local, uncommitted EditorUserSettings; disabling MCP clears managed approval state and drains pending work.
• Source, third-party notices, SBOM, native libraries, and debugger executables were built on GitHub-hosted runners and attested against commit b056990babba629c8505a345ddc5a7a2d845ad6d.
• Build: https://github.com/rankupgames/Unity-Unterm/actions/runs/29296790454
• Attestation: https://github.com/rankupgames/Unity-Unterm/attestations/35175215

Verification

• Unity 6000.3.19 compiled production and test assemblies.
• Focused Claude locator tests: 3/3 passed; focused MCP EditMode suite: 29/29 passed.
• Native Rust tests: 55 passed, 1 ignored; macOS debugger check passed.
• cargo audit, cargo deny, and locked metadata checks passed; only the repository's explicitly allowed pre-existing unmaintained advisories remain.
• GitHub supply-chain, Windows build, macOS build, and provenance-attestation jobs passed.
• gh attestation verify passed for all four native artifacts, notices, SBOM, and source manifest using the repository workflow and exact source digest.

Dependency

This is stacked on #2 so the upstream sync and these behavioral changes remain independently reviewable. Retarget this PR to main after #2 merges.