Add compile-time inline ownership hook (ALLOC8_XXOWNS_INLINE_HEADER)

[emeryberger]

Summary

Adds a third, fastest ownership-dispatch mode to the macOS wrapper: a compile-time inline hook. The consumer defines ALLOC8_XXOWNS_INLINE_HEADER (directly or via the new CMake cache variable) naming a header that provides:

static inline bool alloc8_xxowns_inline(const void* p);

with the same contract as xxowns() — safe on any address, no false positives or negatives.

Why

The runtime xxowns() hook always costs one function call per free/realloc/malloc_size: the weak module-local default definition blocks cross-module inlining under (Thin)LTO, so even a two-load ownership predicate pays call overhead on every operation. Profiling Hoard on larson showed the wrapper layer (entry checks + the xxowns call) at ~25–30% of total cycles.

Because the interpose sources are compiled inside the consumer's target, a compile-time hook inlines straight into replace_free / replace_realloc / replace_malloc_size; the runtime "do we have a hook?" branch disappears and per-allocation track/untrack bookkeeping compiles away entirely. After this change, replace_free's owned-pointer path contains zero calls.

Results (Hoard on M1 Max, 8 threads, runtime hook → inline hook)

benchmark	before	after
larson (ops/s)	185M	211M (+14%)
threadtest (s)	0.85	0.66 (−23%)
linux-scalability (s)	0.127	0.093 (−27%)

Compatibility

• Allocators that don't opt in are completely unaffected (runtime xxowns() and the internal size table are unchanged).
• Consumers may set the define even when building against older alloc8: it is simply ignored.
• Companion Hoard PR: emeryberger/Hoard (ownership-map + size-class optimization series) opts in via hoardownsinline.h.

Testing

• Built into Hoard on macOS arm64; verified via disassembly that the predicate fully inlines (no bl on the owned-free path).
• Multi-threaded stress test (pattern-verified malloc/free/realloc/calloc/posix_memalign with cross-thread frees) passes.
• Full benchmark suite (threadtest, larson, linux-scalability, cache-scratch/thrash) runs correctly.

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