Y2B-220 — Build a tiny diagnostic initramfs

> **Historical record.** The dates, candidate identity, "current" claims, > next steps and permissions below belong to this recorded boundary. See > [current state](../../CURRENT_PLATFORM_STATE.md) for the latest physically observed result.

Depends on Y2B-215 (#11). Offline only; no shell, storage, radio or recovery actions.

Implement a freestanding static ARM EABI /init with direct audited syscall numbers. PID1 opens the console, mounts proc/sysfs read-only, emits bounded diagnostics and periodic heartbeat. An explicit self-test path exits before privileged operations for QEMU user-mode execution.

Acceptance:

No dynamic interpreter/libc dependency; ARMv7 ARM mode, little endian, soft-float.
Deterministic newc/gzip archive with only /init, required directories and console/null device nodes; actual length generates the DTS initrd end.
D08 gzip and unpacked file caps enforced; paths/modes/ownership/timestamps explicit.
QEMU user-mode self-test verifies executable ABI/markers; archive inspection verifies contents. This is not system or hardware boot evidence.

Validation: static ELF32 ARM executable 2,740 bytes; QEMU 10.0.0 Cortex-A7 self-test passes. Deterministic newc is 4,096 bytes, gzip 1,582 bytes. Independent strict archive decoding matches /init exactly; malformed paths, type changes, truncation, extra gzip members and decompression overflow are rejected. Six total validator tests pass.