Point-file JSON validity, interactive-plot readout fix, and plot-style docs

[tbody-cfs]

Three independent fixes off main, found while verifying a plugin's outputs against the #153 → #155 stack (none is caused by that stack; all are pre-existing).

1. Point files are now valid JSON. _RoundingFloat formatted floats with f"{x:#.6g}"; the # alternate form suppresses trailing-zero removal, so a float whose integer part uses all six significant figures was written as 732029. — rejected by json.load. Both committed point files were affected; nothing in the repo reads them back, which is why it survived. Such values now get one extra significant figure (732029.5 — #.6g ends in . exactly when the integer part has six digits, and at #.7g that case always gains a digit after the point). The regenerated reference files differ by exactly the two broken lines. Known limitation, deliberately unchanged: bare NaN tokens remain (Python's json accepts them; allow_nan=False would instead crash writes on real NaN data).

2. Interactive popcon plots no longer crash on mouse-over for unitful fill variables. CoordinateFormatter called float() on array.sel(...).item(), which is a pint Quantity for any quantified field — a DimensionalityError on every mouse move under --show. Invisible until now because both example plot styles fill with dimensionless Q and the only test used an unquantified array. The array is dequantified on construction and the readout now shows the units (z=1.23 [MW]); tested with a quantified array.

3. New docs page plot_styles.rst, a reference for the plot-style yaml keys — previously undocumented beyond incidental notebook usage. Notably: the fill variable's units are appended to cbar_label automatically, so a label which already contains units renders them twice, while contour and axis labels are used verbatim. A pointer comment now sits next to cbar_label in the example style file.

Gates: bare pytest -q 156 passed (includes the doc notebooks); regression_results.py then pytest tests/test_regression_against_cases.py clean; ruff check/ruff format --check clean; mypy cfspopcon at the 58-errors-in-5-files baseline; variable_consistency_checker.py exit 0; docs -n -W, doctest and linkcheck clean.

[MishaVeldhoen]

Looks great!