subset capi: expose plans, mappings, metadata, and C++ ownership

[behdad]

Requires a Skera release containing the public Plan APIs from googlefonts/fontations#2234 . Keep this PR in draft until that release is available and replace the temporary Skera/write-fonts Git pins before merging.

Add HarfBuzz-compatible subset plan creation/reference/destruction/execution, original-to-subset and reverse glyph maps, Unicode-to-original maps, and user data on inputs and plans. Plans snapshot source tables and input settings, support repeated/concurrent execution, and return borrowed core hr_map_t handles that may be retained independently. Metadata replacement/destruction invokes release callbacks outside the lock for reentrancy. Custom no-subset-table selections pass through unchanged, with explicit drops and hint removal taking precedence.

Extend hr-hb-cplusplus.hh with subset input and plan ownership helpers, including both header include orders. Extend Linux CI with the 16 plan/map comparisons, bringing the total to 79, and exercise plan execution/mapping queries in the C example.

These APIs are part of the optional subset feature of harfrust_capi; enable it with cargo build -p harfrust_capi --features subset and link the single harfrust_c library. Headers, examples, and tests live under harfrust_capi.

Validation: all 79 differential subset cases pass, including plan snapshots, mappings, concurrent execution, retained map lifetimes, metadata rejection/replacement/reentry/destruction, and table passthrough/drop precedence. GCC/Clang C99 shared/static and C++11/C++17 ownership consumers pass. Generated headers remain synchronized.

Stacked on #519. Merge order: #544 → #545 → #546 → #519 → #547. The first four PRs use published dependencies and can merge while this PR waits for Skera. Axis instancing, user-specified glyph mapping, and CFF/CFF2/VARC support remain separate work.

Replaces #513; the earlier PR retains its review history.