fix(matrix): wrapped runs enter the font subset

[eli64s]

Summary

Wrapped matrix text shipped without its glyphs. The matrix repair on main wraps cell text, notes, and headers into runs, but the font-surface collector only read the single-line fields. In the default embed mode Ω b d g i s rendered as tofu or fell back to a host font.

Separately, main is red. Four assertions searched the whole SVG for the digits 1200, which timestamps, digests, and font bytes all match. One run spelled them by chance.

Changes

• Font surface: _text_surface collects text_lines, sub_lines, and header lines. Every painted run reaches the subsetter.
• Tofu gate: A wrapped matrix with Ω, ß, Ü, Ø, and μ joins the per-frame cases. A corpus check asserts every rendered codepoint is in the embedded cmap.
• Text-layer assertions: rendered_text joins the painted <text> and <tspan> nodes. The stats-card and chart-frame checks read it instead of the raw SVG, which is the red CI fix.

Validation

Gate	Result
Tofu gate on old collector	2 failed: wrapped case and corpus case
Tofu gate on this branch	22 passed
Matrix frame · font subsetting · stats card · chart frame · chart engine	140 passed
ruff check · ruff format --check · ty check	clean
Full just qa	not run locally; CI runs it here