Phase 2: QEMU aarch64 / armv7 CI

[tap]

Part of Phase 2 — Reach (ROADMAP.md).

Cross-architecture CI under QEMU to validate aarch64 / armv7 (and big-endian behaviour for the endian-agnostic (de)serialization landed in #8).

• qemu-user + cross toolchains (aarch64, armv7) building + running the test suite.
• Confirm big-endian load/store correctness on a BE target.

Demand-gated. This adds standing CI surface; stand it up only when a real cross/embedded target needs it. Note the freestanding profile already exercises the no-OS code paths without QEMU's cost, and the byte-assembly (de)serialization is endian-agnostic by construction (UBSan-clean), so this is verification rather than new capability.

[tap]

aarch64 landed in #20 (branch claude/phase-2-prep-h8q9mz).

• New aarch64-qemu CI job: cross-compiles with aarch64-linux-gnu-g++ and runs the suite under qemu-user via CMAKE_CROSSCOMPILING_EMULATOR. Proves the Raspberry Pi 5 ISA build green on every push and exercises the endian-agnostic (de)serialization on a non-x86 target.
• OscConcurrencyTest is excluded from the emulated run (real threads + loopback sockets are flaky under qemu-user; the native TSan/POSIX legs cover it).
• Verified locally: 98/98 unit tests + the freestanding test pass under qemu-aarch64.

Still open under this issue: armv7 (32-bit) and optionally a real-hardware runner. Retitling/repurposing this issue to those is reasonable.

---

Generated by Claude Code