compiler: add LTAT/RTAT runtime font profile

[mustafa0x]

Summary

Add an explicit --runtime-font mode to the Qt-free OpenType generator.

The mode:

• implies extended variable output;
• emits exactly LTAT at index 0 and RTAT at index 1;
• keeps the normal project-defined axis profile, including Madina's SCLX, unchanged outside runtime mode;
• serializes normalized tatweel values for single, expansion, and alternate extended substitutions;
• validates signed axis ranges, tatweel bounds, and variation delta dimensions;
• produces deterministic output under SOURCE_DATE_EPOCH.

It does not add platform adapters, page APIs, or runtime integration.

Validation

Using madinafont/stdlibconv and the native plugin-loading fix in #6:

• Release Qt-free compiler and Madina plugin builds passed in two independent build directories.
• Standard, extended, and runtime output was byte-identical across the independent builds under SOURCE_DATE_EPOCH=1700000000.
• Standard output SHA-256: 843af0296afd86accdbb7191254689ebf3d1ff489a3012f5d484eb94b71cb318.
• Extended output SHA-256: f5d3ea264ec9c10008875fd94e4859578ee1d84e9ad2e235301e4d69c70c23b9.
• Runtime output SHA-256: 2d05d40f2c23057e148c42efe8cb21e1f92c22d29cc8476f7857dbfc5273c082.
• Standard output retained SCLX [50, 100, 200] and omitted JTST. Its content tables remained byte-compatible with the clean loader-only baseline; head reflects deterministic checksum/timestamps and hhea now uses initialized aggregate metrics.
• --runtime-font produced a 3,256,512-byte font with CFF2, GSUB, GPOS, GDEF, JTST, fvar, and HVAR.
• The only runtime variation axes were LTAT [-20, 0, 20] and RTAT [-20, 0, 20], in that order.
• The runtime GDEF variation store parsed completely with 208 rows and no all-zero rows.
• The generated runtime font passed the standalone C runtime's direct-HarfBuzz conformance suite, including shaping, target-width justification, and base/dynamic CFF2 outlines.

Dependency

• core: use a valid native plugin loading mode #6 fixes the existing native plugin-loading blocker used by the command-line generator. It is intentionally kept as a separate PR.