fix(lint): skip build trees and git-ignored files in check_displayxr_app

[dfattal]

Symptom

After a local Gradle/CMake build of an Android test app, scripts/check_displayxr_app.py walked into AGP's externalNativeBuild output (.cxx/). That output holds the FetchContent'd displayxr-common sources under _deps/, including its tests/. The linter then reported false INV-3.1 errors against displayxr-common's test files. Seen on test_apps/handle/cube_handle_vk_android on 2026-10-11 (#1945 work).

Fix

• All ten directory walks in the script (iter_source_files, MCP pairing, manual-tracking, views-change, vk discipline, Android, manifests, ...) used the same inline EXCLUDE_DIRS check. They now share one is_excluded(path, root) predicate.
• EXCLUDE_DIRS gains .cxx, .externalNativeBuild, .gradle and _deps. build was already on the list.
• The predicate also skips untracked git-ignored paths: one cached git ls-files -z --others --ignored --exclude-standard --directory call per root. Tracked files are never hidden, so a committed source inside an ignored directory is still linted. Outside a git work tree, or with no git, the fixed list is the only filter.

Test

• New hermetic test scripts/tests/test_check_displayxr_app_skip_build_trees.py, wired into lint.yml next to the existing app-linter tests. It covers:
  • a control case where the offending source does trip INV-3.1 in src/
  • each build-tree name being ignored
  • a build tree that cannot mask a real app error
  • the separate vk-discipline walker
  • the fallback when the root is not in a git repo
  • git-ignored files being skipped
  • a tracked file in an ignored directory still being linted
• Mutation-checked: removing the new EXCLUDE_DIRS entries fails 5 cases, and disabling the git-ignore half fails the git case.
• Lint output for test_apps/handle/cube_handle_vk_macos and test_apps/handle/cube_handle_vk_android is byte-identical to main. The existing color and vk_frame linter tests still pass.

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