docs: Update async test coverage status in README

Updated tests/asyncio/README.md to reflect new test additions:
- Added test_environment.py to structure section
- Added description of environment tests
- Updated coverage from ~25 to ~57 async tests
- Marked all coverage goals as completed (✅)

Coverage breakdown:
- 36 baseline async tests
- 17 environment operation tests
- 4 version edge case tests
Total: 57 async tests

Author: tony

tests/asyncio/README.md

@@ -10,6 +10,7 @@ tests/asyncio/
 ├── test_basic.py                # Basic async functionality tests
 ├── test_acmd.py                 # Pattern A (.acmd() methods) tests
 ├── test_tmux_cmd.py             # Pattern B (tmux_cmd_async) tests
+├── test_environment.py          # Async environment variable operations tests
 ├── test_hybrid.py               # Tests for both patterns working together
 └── test_docstring_examples.py  # Verification tests for docstring examples
 ```
@@ -33,6 +34,16 @@ Tests for direct `tmux_cmd_async()` execution:
 - Direct async command execution
 - Socket isolation with `-L` flag
 - Async-first architecture patterns
+- Version checking functions and edge cases
+
+### test_environment.py
+Tests for async environment variable operations:
+- Session-level environment operations (set, unset, remove, show)
+- Server-level global environment operations (with `-g` flag)
+- Concurrent environment modifications
+- Environment isolation between sessions
+- Edge cases (special characters, empty values, long values)
+- Global vs session-level precedence
 
 ### test_hybrid.py
 Tests for using both patterns together:
@@ -147,17 +158,17 @@ Following CPython's asyncio test patterns:
 
 ## Coverage Goals
 
-Current coverage: ~25 async tests
+Current coverage: ~57 async tests (36 baseline + 17 environment + 4 version edge cases)
 
 Target coverage:
 - ✅ Core async functionality (tmux_cmd_async, get_version)
 - ✅ Pattern A (.acmd methods)
 - ✅ Pattern B (tmux_cmd_async)
 - ✅ Concurrent operations
 - ✅ Error handling
-- ⏳ AsyncEnvironmentMixin methods (pending)
-- ⏳ Version checking functions (pending)
-- ⏳ Edge cases and error conditions (pending)
+- ✅ Environment operations (17 tests covering all operations)
+- ✅ Version checking functions (including edge cases with mocking)
+- ✅ Edge cases and error conditions (special chars, boundaries, race conditions)
 
 ## Best Practices