engine: add standalone native runtime

[mustafa0x]

Summary

Add an opt-in, standalone native C ABI for consuming DigitalKhatt runtime fonts without linking Qt, MetaPost, the authoring application, or platform UI code.

The ABI-v1 kernel:

• loads a font from a file or copied memory;
• shapes one independent RTL Arabic UTF-16 line;
• optionally applies the font's JTST target-width pass;
• returns glyph IDs, UTF-16 clusters, positioning, and normalized LTAT/RTAT values;
• emits the selected dynamic CFF2 foreground outline through renderer-owned callbacks;
• propagates callback cancellation or failure;
• supports concurrent calls on an immutable engine;
• exposes explicit status codes and opaque ownership.

All requests and outputs are size-versioned. Glyphs are read through accessors rather than a borrowed fixed-stride array, allowing records to grow without changing the ABI contract.

Parent integration is disabled by default through DIGITALKHATT_BUILD_NATIVE_ENGINE.

Font contract

The runtime admits only the deliberately narrow native profile:

• CFF2, GSUB, GPOS, GDEF, JTST, fvar, and HVAR;
• exactly two variation axes;
• LTAT at index 0 and RTAT at index 1;
• both axes centered at zero with negative and positive ranges.

Validation

Against the corrected deterministic runtime font produced by #7:

• Release shared build and pure-C ABI compilation passed.
• ASan and UBSan passed.
• Natural, stretched, shrunk, and saturated line output matched direct HarfBuzz exactly.
• Glyph IDs, clusters, advances, offsets, tatweel values, and total widths matched.
• Base and dynamic CFF2 outline command streams matched direct HarfBuzz.
• File/memory parity and concurrent shape/outline calls passed.
• Every required-table omission and invalid axis contract was rejected.
• Empty outlines, append-only request/output records, append-only path sinks, and callback cancellation passed.
• The shared-library export surface contains only the ABI-v1 symbols.

Deliberate non-goals

• FeatureJustifier or Mushaf page policy
• Page composition
• Tajweed/COLR/CPAL painting
• Line breaking and layout UI
• Android/iOS adapters
• MetaPost authoring or font generation

The line operation is documented as a low-level JTST primitive; it is not presented as the canonical Madinah page-layout contract.

Dependency

This runtime consumes the LTAT/RTAT font profile produced by #7. Keeping the producer and consumer changes separate makes both reviews smaller and keeps the C ABI out of compiler internals.