feat: interrupt-driven I/O and SD persistence for OS mode

[ds1sqe]

Summary

Make the bare-metal editor durable and responsive under load: GIC-400 interrupt
bring-up (UART RX feeding the input ring instead of polling) and a persistence store
on the SD card satisfying the storage-neutral atomicity contract.

Part of #788 (phase OS-4).

Proposed change

• Interrupts: GIC-400 init (distributor + CPU interface, EL1 vectors); PL011 RX
  interrupt feeding a ring buffer consumed by the scheduler loop; generic-timer
  compare interrupt as the park/wake deadline source.
• Storage: EMMC2 SD-card driver (single block read/write first).
• Store: persistence backend satisfying the restated PS1 atomicity contract
  (docs: spec deltas enabling freestanding and multi-target platforms #789). Decision recorded in this issue: FAT32 (host interop — the SD card stays
  readable on any machine) vs a small log-structured store (cleaner atomic-generation
  fit, no directory-rename emulation). Evaluate both against PS1 and pick one.
• State lifecycle: session save on shutdown (PS2) and restore on boot, surviving
  power cycles.

Acceptance criteria

• Input path is interrupt-driven end to end; no UART polling in the steady state
• Editor state survives a hard power cycle (save, power off, boot, restore)
• Chosen store's conformance to the restated PS1 argued in the storage doc
• Timer interrupt drives park/wake deadlines (no timer polling)

Related

• Part of proposal: OS mode — reovim as a bare-metal OS (Raspberry Pi 4) #788
• Depends on feat: single-seat bare-metal editor image (OS-mode MVP) #793, docs: spec deltas enabling freestanding and multi-target platforms #789