Use face-owned glyph-0 outlines for uncovered text

Valid uncovered BMP and astral codepoints now render through the selected face's glyph-0 (.notdef) outline instead of becoming solid text-colored cells; empty or unusable fallbacks intentionally contribute no ink. Registered-font ink metrics follow the same behavior while preserving advances and malformed-input handling. Added sans/mono, shaped/plain, registered-font, and synthetic-font regressions, plus aligned documentation and rendering comments.