The cart-base map at 0x10000000 (so absolute ROM-asset symbols are live host
pointers) was Windows-only; the #else branch was a stub that logged
"not implemented" and fell to the degraded heap-copy path.
- Factor the post-map work (memcpy + pcmodels/pccg sidecar load + D55 RLE
header fixup) into romdataFinishCartMap(), shared by both paths.
- POSIX #else: anonymous mmap at CART_BASE with MAP_FIXED_NOREPLACE where
available; on any failure munmap + fall through to the identical heap copy.
- romdataDestroy() munmaps on POSIX (length tracked in mappedLen).
Verified on Windows: builds 242/242, -level_09 boots crash-free (identical
behaviour with/without the refactor). Linux compile + runtime verification
owed — see docs/dev/LINUX-PORT.md. No behaviour change on Windows.
