S8: arm64 mmio_read32 returns 0 / mmio_write32 no-op — every ARM64/Pi driver inert

Stub-burndown register S8 (P3). File: kernel/src/arch/arm64/arm64.home.

Blocks Phase 7b entirely. Prerequisite: volatile MMIO semantics defined in the language spec (A3, [MASTER_PLAN §6]).

Tracked under the Raspberry Pi 5 media-center epic #113 (compiler A6 breakdown: #51 #52 #53; S8 de-stub: #60; real build.sh rpi5: #54).