runtime: preserve malloc allocations until free

C malloc storage has explicit lifetime: it must remain allocated until
free even when no GC-visible pointer references it. Treating it as an
ordinary NoPtrs allocation breaks bare-metal C object graphs, while
conservatively scanning arbitrary C bytes creates false Go roots.

Add allocManual/freeManual so collectors can represent pointer-free,
explicitly managed storage. Block GC keeps these objects permanently
marked and releases their blocks on free; Boehm uses atomic uncollectable
allocations; leaking and custom collectors provide equivalent behavior.
Wasm keeps its allocation map only for validation and sizes, and WASIp2
realloc now copies min(oldSize, newSize).

Bump the Boehm library cache version because enabling atomic
uncollectable allocations changes its compiled flags and exported API.

Also handle zero-size and overflowing allocations, serialize allocation
registries, reject Go finalizers on manual storage, and add CGo regressions
for C pointer graphs, hidden until-free allocations, repeated free/reuse,
and allocation edge cases.

testdata/cgo-realloc/out.txt

@@ -0,0 +1 @@
+realloc preserves contents: 42