Mark crates as miri-compatible and wire a just miri CI job #6

Context: just miri exists but isn't in CI, and most crates aren't explicitly marked as miri-compatible.

Scope:

1. Add a miri_compatible parameter to rust_library in build/rust.bzl, default True.
2. Set False on crates with inline asm, MMIO, or hardware access (at minimum: riscv, trap, uart-16550, cpu-local, unwind, backtrace, panic-unwind, abort).
3. Add a test-miri CI job that runs just miri over the compatible set.
4. Start as continue-on-error: true for a week to collect failures; then make required.

Initial miri-compatible targets to verify: arrayvec, fastrand, ecma-119, fdt, wavltree, range-tree, sharded-slab, util.

Acceptance: miri CI job runs; compatible crates pass.