fix: preserve fixed boxes with None alignment

[MbappeWU]

Fixes #294.

With stralign=None, a pretty table sizes its border for the widest cell but leaves shorter rows unpadded. Preserve the cell and header contents while adding the space required by closed, fixed-width text boxes. Use the existing TableFormat row/line metadata to identify those boxes; plain output and flexible markup keep their existing no-alignment behavior.

Tests cover whitespace, headers, multiple columns, multiline rows, separators/indexes, custom boxes, open/callable lines, and header-only formats. Numeric parsing is unchanged.

LLM assistance: Codex. The independent reviews were performed by agents; no human review is claimed.

Validation and the existing lint blocker

• Full suite with NumPy, pandas and wcwidth: 388 passed, 1 skipped. Bare environment: 329 passed, 60 skipped. Both include module/README doctests. The final focused tests give 3 failures and 3 passing controls against exact upstream 268615a, and all 6 pass with this patch.
• Original pytest matrix passed all 15 Linux/Windows/macOS × Python 3.10–3.14 jobs, each 388 passed and 1 skipped. Fork CI head affc1d8 differs from source 339649d only by a manual trigger in lint.yml; all 26 tracked entries and the original jobs/steps were compared.
• Project-pinned Ruff 0.15.4 lint and formatting pass. The unpinned lint job fails under Ruff 0.16.10: all 13 tracked Python files produce the same 272 diagnostics on the exact base and this branch. Ruff 0.16 expanded defaults and Markdown formatting; existing PR Bump ruff #454 handles that upgrade. The failing job is not reported as passed, and this patch does not weaken rules or change workflows.
• A private integration with existing PR Bump ruff #454 merged cleanly, passed Ruff 0.16.10 lint/format, and ran 388 passed / 1 skipped. This tests both patches together; the current standalone branch retains the disclosed lint blocker and is submitted as a draft.
• An isolated installed wheel and source matched across 17 public API cases, including 7 box geometry cases and baseline-equal flexible-format/numeric controls. Three real CSV/JSONL CLI invocations matched the base. None alignment was tested through the Python API; the CLI has no invented None flag. Wheel/sdist tracked runtime modules matched the final source byte for byte.