Read the new VARC MultiItemVariationStore format #299

HarfBuzz main commit bd079077d2 (2026-10-02, after the 14.5.0 release; boring-expansion-spec PR #175) changed the VARC MultiItemVariationStore format: the region count is a u32, the delta-set INDEX is reached through an Offset32, and offsets are relative to the region list. HarfBuzz migrated its own test fonts (varc-6868, varc-ac0*) to the new format.

sigilbuzz reads the 14.5.0 format. After #298, a VARC table in the new format is treated as absent (glyphs draw from glyf/CFF/CFF2, as HarfBuzz 14.5.0 does), instead of failing every glyph.

To do once fontTools and a HarfBuzz release write and read the new format:

• Read both formats (detect by layout, or by the table's version if the spec bumps it).
• Add the migrated HarfBuzz test fonts as fixtures and compare outlines with the HarfBuzz release that supports them.
• sigilbuzz-subset re-emits VARC bytes; check that it round-trips the new format too.