fix: guard _find_package_dir against missing module paths

[michaeljabbour]

Summary

When a module's cache directory is missing (e.g., never cloned), _find_package_dir() calls iterdir() on a non-existent path, causing an unhandled FileNotFoundError that crashes session initialization.

• Add existence check at the top of _find_package_dir() — returns None when module_path doesn't exist
• The caller (_validate_module) already handles None by raising a clear ModuleValidationError
• Converts an opaque FileNotFoundError crash into an actionable error message

Root Cause

The source resolver computes expected cache paths via sha256(url)[:16] and returns them without verifying the directory exists. When a module's cache entry is missing (e.g., local bundles whose git-sourced modules weren't pre-cached), the loader crashes in _find_package_dir at the iterdir() fallback.

Test Plan

• 7 new tests in tests/test_loader.py covering:
  • Non-existent path returns None (core regression)
  • No FileNotFoundError raised (anti-regression)
  • Hash-style cache path pattern handled
  • Empty directory returns None
  • Happy path still works
  • Caller produces ModuleValidationError (not FileNotFoundError)
  • Error message includes module ID and path

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