Implement Spike B step 1 — the highest-risk reconstruction step, not covered
by Spike B's synthetic corpus (where Unicode was known by construction).
GlyphUnicode.cpp exercises the §4.1 step 1 priority ladder on a real PDF's
embedded font:
- Step 1 (ToUnicode): parses the CMap's beginbfchar sections via QPDF into a
code→Unicode map. Confidence 1.0 (authoritative).
- Step 4 (cmap fallback): for glyphs the ToUnicode didn't cover, walks the
embedded font's cmap via FreeType (FT_Get_First_Char/Next_Char) to
reverse-lookup gid→codepoint. Confidence 0.7.
- Assembles per-glyph mappings with provenance + confidence — the same
confidence the production UI uses to underline low-confidence characters.
Gate MET on /usr/share/doc/shared-mime-info/shared-mime-info-spec.pdf: 50
glyphs via ToUnicode (conf 1.0), 37 via cmap fallback (conf 0.7), 1
unmapped, coverage 98.4%, avg confidence 0.818. Verified clean under
ASan+UBSan. Result in docs/spike-results/0006-spike-b1-glyph-unicode.md.
Findings: (1) the ToUnicode path works on real embedded fonts (50/62 at
confidence 1.0). (2) the FreeType cmap fallback recovers the rest (37 at
0.7). (3) the confidence signal is meaningful — 0.7 vs 1.0 falls out of
which ladder step resolved the glyph. (4) QPDF+FreeType cover steps 1 and 4;
steps 2-3 (Encoding/Differences/AGL, CIDSystemInfo) and bfrange/CID-keyed
ToUnicode parsing remain M4 work.
Combined with Spike B (F1=0.963), the reconstruction story is de-risked:
the pipeline reconstructs paragraphs from glyph runs, and glyph runs
resolve to Unicode via the ladder. The M4 text-editing milestone has its
foundation.
CI: add Spike B1 to the spike-gates job (shared-mime-info for the test PDF).
Signed-off-by: ai-ad4 <ai-ad4@users.noreply.gitea.lm.je>
ai-ad4