Add fixed units-per-tick scaling for bar charts

[GreedyC]

Summary

Termgraph currently rescales every chart to the current dataset maximum, making repeated command outputs hard to compare. This adds fixed units-per-tick scaling requested in #108.

• Add --scale N and Args(scale=N) for horizontal, stacked, and vertical bar charts. Each positive value rounds up to a whole tick; zero stays empty.
• Keep existing automatic scaling when the option is absent. Reject non-finite or negative data and incompatible chart modes instead of silently changing the baseline.
• Document the CLI and Python API. Explicitly retain Ruff's established E4/E7/E9/F rule set so current Ruff releases do not add unrelated default-rule failures.
• In a separate commit, make two existing file-input tests portable on Windows: those tests reopened a still-open NamedTemporaryFile, which failed in the preflight run.

Verification

• 71 tests, Ruff, and mypy pass locally on Python 3.9 and 3.14.
• The fork-only cross-platform preflight passes on Linux, Windows, and macOS with both Python versions (6/6). Its branch has the same project files as this PR; only its workflow file differs.
• The repository's existing CI passes.
• Built and checked the source distribution and wheel.

Code and tests were prepared with Codex assistance.

Closes #108.

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