fix(render): cover SVG symbol glyphs with the embedded broad-coverage face

svg_font_database loaded only the Liberation faces, which stop at
Latin/Greek/Cyrillic. An SVG text run with a symbol glyph (star ratings,
bullets, checkmarks - reported as U+2605) therefore had no face covering
it anywhere in the database: usvg's per-glyph fallback search found
nothing, rendered the .notdef tofu, and warned per character on every
rasterized SVG text run.

Load the embedded DejaVu Sans face (SYSTEM_FONT_BYTES) into the SVG
database too - the same broad-coverage face the HTML text engine already
keeps for system-ui. Family selection is unchanged: Liberation stays the
sans/serif/mono default, DejaVu only extends what the fallback search
can find, keeping rendering deterministic (no system font scan).

The regression test installs a log collector (log is now a
dev-dependency so it resolves to the same instance usvg logs through,
and set_max_level is required - the facade default filters everything
off) and asserts a star text run rasterizes without a missing-glyph
warning, with an unassigned-codepoint control so the test fails loudly
if the logger wiring breaks.

Fixes #701.