Behavior-neutral prep on top of the inline-load parity milestone:
* JitCache caches mem.fast_mem() (resolved once — the mapping is invariant for
a run) so compiled blocks build their MemEnv without a virtual call per
execution. MemEnv::from_fast(mem, Option<FastMem>) is the non-virtual
constructor. Measured impact is within noise (96.5 -> 96.4 MIPS) but it
removes redundant per-block work.
* FastMem/MemEnv gain mmio_count: a pointer to GuestMemory's monotonic MMIO
access counter. Unused for now; the upcoming superblock-chaining JIT will
sample it across a block boundary to stop chaining on an MMIO touch
(preserving the interpreter's fine-grained MMIO ordering).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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