fix(vfs): handle ./ prefix in path resolution

[chaliy]

Summary

• Normalize . and .. path components in Interpreter::resolve_path, glob directory resolution, and PosixFs methods
• Ensures ./filename resolves identically to filename relative to cwd across all VFS operations (ls, cat, redirections, test builtin, globs)

Changes

• Interpreter::resolve_path: Now calls normalize_path on the joined result, fixing redirections (> ./file), script execution (./script.sh), and the test builtin ([ -f ./file ])
• Interpreter test-command lambda: Same normalization for inline path resolution in [/test
• glob::expand_glob: Normalizes the directory path when pattern has a relative prefix like ./
• PosixFs: All FileSystem trait methods now normalize input paths before delegating to the backend, ensuring custom FsBackend implementations work correctly with . and .. components

Test plan

• test_dot_slash_prefix_ls — ls ./file works
• test_dot_slash_prefix_glob — echo ./*.html works
• test_dot_slash_prefix_cat — cat ./file works
• test_dot_slash_prefix_redirect — echo x > ./file works
• test_dot_slash_prefix_test_builtin — test -f ./file works
• test_posix_normalize_dot_slash_prefix — PosixFs handles ./ paths
• test_posix_normalize_preserves_semantics — parent-exists checks still work
• All 2036 existing tests pass

Closes #1114