Implement Spike C: grow an existing embedded subset font with a new glyph
using hb-subset and verify the result, exercising §4.2 resolution ladder
step 1.
SubsetGrowth.cpp runs a three-step scenario with the HarfBuzz subset C API
and FreeType:
1. Build the 'originally embedded' subset of DejaVu Sans with glyphs for
'Hello' (H,e,l,o).
2. Grow it by adding U+03A9 (Ω) — the character the user typed that wasn't
in the original subset. Produces a NEW font; original untouched (§4.2:
'never mutate a font program in place').
3. Verify: grown subset contains the glyph, cmap maps U+03A9 → gid, FreeType
loads its outline (it renders); original subset does NOT contain it.
Gate MET: original 5 glyphs/4300 bytes → grown 6 glyphs/4488 bytes, new
glyph present and renders. Verified clean under ASan+UBSan. Result and
findings in docs/spike-results/0003-spike-c-subset-growth.md.
Findings: (1) hb-subset growth works and is cheap (+188 bytes/glyph). (2)
HarfBuzz subset API changed across versions — HB_SUBSET_SETS_DROP doesn't
exist in 10.2; pin the baseline (vcpkg manifest does). (3) FT_Get_Char_Index
+ FT_Load_Glyph is the right render-verification pair; the real viewer
check (Acrobat/Preview/Chrome) remains a release gate (§8.3). (4) the
'never mutate in place' rule is honoured by construction (hb_subset_or_fail
returns a new face).
CI: add Spike C to the spike-gates job (libharfbuzz-dev, libfreetype-dev,
fonts-dejavu-core); fails the build on regression.
Signed-off-by: ai-ad4 <ai-ad4@users.noreply.gitea.lm.je>
ai-ad4