fix(matrix): named refusals and whole-pixel floors; POST body reaches every field

[eli64s]

Summary

Seven bugs survived the matrix repair on main because nothing tested for them. Two are user-facing: a headline chip wider than the masthead ran outside the frame with no error, and POST /v1/compose accepted 24 of the 46 ComposeSpec fields and ignored the rest in silence. All seven now have tests.

Changes

• Headline chip: Raises MatrixCapacityError with the space needed and the space available. Next to a title, the chip leaves room for the gap and the title's shortest truncation.
• Declared widths: A width narrower than the column's content is widened to fit, with a declared-width diagnostic. The matrix refuses only when the columns cannot fit 900px at their narrowest.
• Whole-pixel widths: text_floor rounds up. A column set to its own minimum used to truncate the text that minimum came from, because the two differed by a fraction of a pixel.
• Fitter: truncate_to_width returns nothing when even … does not fit. fit_line and fit report the overflow.
• Axis ticks: The axis reads 1k · 2k · 3k instead of 1000k. Millions stay M. _fmt_tick is replaced by format_compact_number with a lower_k option.
• Unused config: metric_text_x, text_inset, and strip_metric_text_x are deleted. Nothing read them.
• Specimen fixtures: 13 fixtures pointed at a moved directory, so they skipped instead of failing. The suite now fails when a specimen is missing.
• POST body: ComposeRequest gains 13 fields including font_mode, on the inline and envelope paths. Unknown keys return in warnings, or X-HW-Warning for svg. The other 13 fields are internal and listed in tests/serve/test_compose_body.py, so adding one forces a choice.
• Housekeeping: _test_*.svg ignored, connectors example regenerated, corpus re-recorded.

Validation

Gate	Result
Seven new tests against main	7 failed
Matrix layout · input · frame · spatial	238 passed
Specimens	127 passed, 0 skipped (was 13 skipped)
POST body contract	14 passed
HTTP · surfaces · integration · MCP	717 passed
ruff check · ruff format --check · ty check	clean
Full just qa	6,328 passed, 105 skipped (was 6,289 / 118)

Nothing visible moved. 14 measurements shifted by at most 1px and the connectors asset by 0.05px.

Notes

• format is reported but ignored on inline svg and json. It works on envelope and report, which serve a file at url.