Replace the growable ArrayList(u8) flat buffer (reallocs/MOVES on growth) with a
std.heap.ArenaAllocator. Each allocBytes is a separate arena allocation that never
moves and is freed wholesale on deinit -- no per-object free, no cap, no fixed
buffer. Addr is now the allocation's ABSOLUTE host pointer (@intFromPtr), not an
offset, so a flat-memory pointer and an FFI-returned host pointer are the same kind
of value -- the FFI bridge (4D.1) passes them to/from libc with zero translation and
no per-call pinning (the moving-buffer hazard is gone by construction).
readWord/writeWord/bytes deref the absolute pointer with a null-check bail (the
malformed-IR / null-deref safety contract). Dropped the offset-based upper-bounds
check (can't bound an absolute pointer; Frame.bad_ref still catches the dominant
malformed-IR vector) and the test-only mark/reset (arena has no reset-to-mark; the
VM never used them outside tests).
697/0 both gates + all unit tests (rewrote the two Machine tests). Pure refactor, no
comptime behavior change.
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