Title text: fontconfig fallback for codepoints outside the primary font

[nikicat]

Fixes #96.

Problem

Both the ab_glyph and the new skrifa title renderers load a single primary font (whatever fc-match returns for the GNOME titlebar-font preference, defaulting to Cantarell) and lay out every codepoint with that one font. Any codepoint not in the primary's charset — emoji, CJK, most symbol ranges — silently becomes .notdef, so its advance is consumed but no real glyph is drawn.

Most visible on GNOME/Mutter Wayland, which forces CSD on clients, so every titlebar Rio / Alacritty / winit-based app paints is affected. A title like hello 😀 你好 ★ renders with three identical hollow boxes.

Reproduction (verified empirically): on the pre-fix code, '★' (U+2605) and '█' (U+2588) — both absent from Cantarell — produce byte-identical pixmaps, because both are drawn as the same .notdef glyph.

Fix

New src/title/font_fallback.rs: for each title char not covered by the primary or any already-loaded face, run fc-match -f '%{file}' :charset=HEX, mmap the file once, and cache it (deduped by path; seen chars not re-queried). Discovery happens in update_title, not in render.

Both backends now parse the primary + fallback faces each render and pick the first face that has a glyph for each char (select / select_face). Layout rules:

• Line metrics come from the primary, so fallback faces with wild ascent/descent don't grow the title bar.
• Per-glyph advance comes from the covering face.
• Kerning is only applied between adjacent glyphs that came from the same face.

crossfont has the same single-face limitation but a different code shape (it owns the rasterizer); leaving it as a follow-up.

The previously-warn!-level "Ignoring out of range pixel" log in the ab_glyph renderer is now debug! — it fires on the fallback path whenever a fallback glyph's outline extends past the primary's height bound, which is expected.

Tests

Two regression tests per backend (in ab_glyph_renderer.rs and skrifa_renderer.rs):

• loads_fallback_for_missing_codepoint — bypasses the system font, sets title '★', asserts a fallback face was loaded. This is the structural invariant; the bug was that fallbacks would always be empty.
• no_fallback_for_ascii_title — sets title "hello", asserts no fallback lookups happened.

Verified on the pre-fix code (separately) that two distinct missing codepoints produce byte-identical .notdef pixmaps — the smoking gun. The committed tests use the structural assertion rather than a pixmap comparison because the latter depends on font specifics (a .notdef that renders the hex codepoint inside the box would mask the bug).

All four feature configurations build clean (ab_glyph, skrifa, crossfont, default) and tests pass on the touched backends:

ab_glyph: 32 passed
skrifa:   38 passed

(Pre-existing clippy lints in crossfont_renderer.rs and a few mismatched_lifetime_syntaxes warnings in parts.rs / theme.rs are unrelated and untouched here.)

Downstream

Downstream user-visible report: raphamorim/rio#1622