capi: collect nominal cmap Unicode coverage

[behdad]

Chromium's optional subset fuzzer calls hb_face_collect_unicodes to populate its input Unicode set. Add the corresponding hr_face_collect_unicodes entry point and HarfBuzz compatibility alias, using the selected cmap subtable's raw character codes and preserving existing set entries.

Format 0 iteration is implemented upstream in Fontations #2265, and coverage for that format depends on a release containing that fix. Insert format-13 constant-glyph ranges directly, bounded by valid glyph IDs and the Unicode range, and exclude format-4's sentinel. Symbol/Mac Roman lookup remapping and variation selectors do not change the collected character codes.

Validation:

• 152 C API tests, workspace Clippy (all features/targets), and Rust 1.85 check pass.
• With published read-fonts 0.45: 491 coverage sets match HarfBuzz 14.6.0, with the two format-0 faces excluded; two format-13 sets are independently verified.
• Corpus comparison with Fontations #2265 patched in: 493 coverage sets match HarfBuzz 14.6.0; two format-13 faces independently match their complete constant-glyph ranges. HarfBuzz incorrectly clips these ranges as though glyph IDs incremented within each range; this implementation keeps the full coverage.
• GCC/Clang C99 linkage and C++17 header checks pass.

Adds a reusable ctypes comparison script and regression tests for additive collection, empty/null handles, inverted sets, and a format-13 last-resort font.

Closes the optional subset-fuzzer API gap in #520; Chromium build integration is tracked separately there.

[dfrg]

The generic Fontations iterator does not implement format 0.

Thanks, seems like this should be fixed in fontations.

[behdad]

Implemented format 0 iteration upstream in Fontations #2265, adding Cmap0::iter() and the generic subtable iterator dispatch. The upstream regression checks all 256 stored character codes, including glyph zero and the final entry.

Removed the format 0 fallback and its coverage cases from this PR. Format 0 now follows the generic iterator, so collecting its coverage depends on a read-fonts release containing the upstream fix.

Validation: 152 C API tests pass with the published dependency. With the upstream fix patched in, the full corpus has 493 matching coverage sets and two independently verified format-13 sets.