fix: allow degraded compositor installs

[avifenesh]

Summary

• distinguish installer prerequisites from optional desktop/compositor capabilities in the final doctor step
• allow successful installs to exit 0 when MCP registration, AT-SPI tree extraction, and development input are ready but window targeting is unavailable
• keep missing core prerequisites as hard failures
• preserve the same behavior when jq is unavailable
• add structured and raw-fallback regressions for both degraded-success and hard-failure cases

Root cause

run_doctor treated every entry in readiness.blockers as an installation failure. That array also includes missing window introspection or exact-focus support, which may be an inherent limitation of a partially supported compositor rather than a failed installer step.

The installer now keys its exit status to the three capabilities required for a usable core installation: MCP tool registration, AT-SPI tree extraction, and development input. If those are ready, remaining blocker entries are reported as unavailable platform capabilities and the install succeeds. Missing core capabilities still return failure and remain in the final failure summary.

Validation

• bash -n install.sh scripts/install_sh_test.sh
• installer regression suite (12/12)
• cargo fmt --all -- --check
• cargo check --locked --all-targets
• cargo clippy --locked --all-targets -- -D warnings
• cargo test --locked --no-fail-fast (237 library tests, 7 helper tests, 1 integration test)
• RUSTDOCFLAGS="-D warnings" cargo doc --locked --no-deps --document-private-items
• cargo publish --dry-run --locked
• scripts/mcp_safety_check.py --binary target/debug/computer-use-linux
• node scripts/zod-check/check.mjs --command target/debug/computer-use-linux
• npm pack --dry-run

Closes #84