feat: add path completion, LCP-based suggestions, and validator tests

- Refactored `FalyxCompleter` to support longest common prefix (LCP) completions by default.
- Added `_ensure_quote` helper to auto-quote completions containing spaces/tabs.
- Integrated `_yield_lcp_completions` for consistent completion insertion logic.
- Added `_suggest_paths()` helper to dynamically suggest filesystem paths for arguments of type `Path`.
- Integrated path completion into `suggest_next()` for both positional and flagged arguments.
- Updated `argument_examples.py` to include a `--path` argument (`Path | None`), demonstrating file path completion.
- Enabled `CompleteStyle.COLUMN` for tab-completion menu formatting in interactive sessions.
- Improved bottom bar docstring formatting with fenced code block examples.
- Added safeguard to `word_validator` to reject `"N"` since it’s reserved for `yes_no_validator`.
- Improved help panel rendering for commands (using `Padding` + `Panel`).
- Added full test coverage for:
  - `FalyxCompleter` and LCP behavior (`tests/test_completer/`)
  - All validators (`tests/test_validators/`)
- Bumped version to 0.1.80.

tests/test_validators/test_key_validator.py

@@ -0,0 +1,18 @@
+import pytest
+from prompt_toolkit.document import Document
+from prompt_toolkit.validation import ValidationError
+
+from falyx.validators import key_validator
+
+
+def test_key_validator_accepts_valid_keys():
+    validator = key_validator(["A", "B", "Z"])
+    for valid in ["A", "B", "Z"]:
+        validator.validate(Document(valid))
+
+
+@pytest.mark.parametrize("invalid", ["Y", "D", "C", "", "1", "AB", "ZB"])
+def test_key_validator_rejects_invalid(invalid):
+    validator = key_validator(["A", "B", "Z"])
+    with pytest.raises(ValidationError):
+        validator.validate(Document(invalid))