Close #66: pipeline hygiene - local import, duplicated constant, toggle validation, set rebuild hoist

[prekabreki]

What changed

Four pipeline-hygiene cleanups: moved local import math to module top, defined WRAPPABLE_INDENT_UNITS in a single literal, added __post_init__ validation on FormatToggles.{font_size,line_spacing}, and hoisted the per-iteration used_hexes.upper() set rebuild in assign_colors.

Files

• vo_format/cold_read.py — hoist import math to module-level imports
• vo_format/models.py — single-definition WRAPPABLE_INDENT_UNITS; FormatToggles.__post_init__ validation
• vo_format/colors.py — maintain used_hexes_upper alongside used_hexes
• tests/test_toggles.py — 8 new tests for __post_init__ validation

Assumptions made

• line_spacing argparse in cli.py has no explicit choices= constraint; used > 0 as the minimum sense check for a multiplier value.
• font_size accepted values are {12, 14, 16, 18} per cli.py's choices= list.

Uncertainties

none

Test

./.venv/Scripts/python.exe -m pytest tests/ -q — 437 passed, 2 skipped

Closes #66