Add environment variable mutexes for test isolation

[skypher]

Summary

Adds global mutexes for PATH and HOME environment variables to prevent race conditions in parallel test execution.

Problem

Tests that modify environment variables using unsafe { std::env::set_var } affect the entire process. Even with per-module test mutexes, these modifications can race when tests run in parallel, causing non-deterministic failures.

Solution

Created src/test_utils.rs with global environment variable mutexes:

• lock_path_mutex() - Serializes all PATH modifications
• lock_home_mutex() - Serializes all HOME modifications

Updated 29 test files across the codebase to acquire these mutexes before modifying environment variables.

Results

Test Failure Reduction:

Before: 17-20 failures (parallel run)
After:  14 failures (parallel run)
Reduction: ~20-30% fewer failures

Sequential Tests:

All 207 tests pass with --test-threads=1 ✅

Remaining 14 Failures

The remaining parallel test failures are due to deeper architectural issues:

1. Global Index State: Multiple tests modify the same global index singleton
2. Disk Cache Sharing: Tests write to overlapping cache directories
3. Timing Dependencies: Some tests have race conditions in async operations

These require significant refactoring:

• Making index state per-test instead of global
• Using unique temp directories per test
• Better async test synchronization

Files Changed

New:

• src/test_utils.rs - Global environment variable mutexes

Updated (29 files):

• src/lib.rs - Adds test_utils module
• src/index/*.rs - 5 files with PATH guards
• src/logic/**/*.rs - 3 files with PATH guards
• src/sources/*.rs - 2 files with PATH guards
• src/install/*.rs - 5 files with PATH guards
• src/events/mod.rs - PATH guards
• src/state/app_state.rs - HOME guards
• src/theme/**/*.rs - 3 files with HOME guards

Dependencies

This PR builds on top of:

• PR Fix test mutex poisoning issues #2 (mutex poisoning fixes)

Testing

# Sequential (all pass)
cargo test --lib -- --test-threads=1

# Parallel (193/207 pass)
cargo test --lib

# With limited parallelism (better success rate)
cargo test --lib -- --test-threads=4