P12.OP6 (miroir-zc2.6): Document arm64 support deferral to v1.x+

This bead remains open as a placeholder. ARM64 support is explicitly
deferred to v1.x+ per Plan §15 Open Problem #6. No current demand
justifies the CI complexity; fleet is all amd64.

When prioritized: cross-compile for aarch64-unknown-linux-musl,
build multi-arch Docker manifest, add arm64 CI test runs.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

notes/miroir-zc2.6.md

@@ -0,0 +1,50 @@
+# P12.OP6: ARM64 Support (Deferred to v1.x+)
+
+## Status: DEFERRED
+
+This bead remains open as a placeholder for future arm64 support. Per Plan §15 Open Problem #6: "Not planned for v0.x. Added when K8s ARM node support is required."
+
+## Current State (2026-05-08)
+
+### Architecture
+- **Target**: `x86_64-unknown-linux-musl` only
+- **CI**: GitHub Actions `ubuntu-latest` (amd64 runners)
+- **Fleet**: All amd64 (iad-ci cluster, ardenone nodes)
+- **Helm**: Architecture-agnostic (no changes needed when arm64 is added)
+
+### Build Artifacts
+- `miroir-proxy`: Static musl binary (amd64 only)
+- `miroir-ctl`: Control plane binary (amd64 only)
+
+## When Prioritized (v1.x+)
+
+### Required Changes
+
+1. **CI Pipeline** (`.github/workflows/test.yml` or separate build workflow):
+   ```yaml
+   - name: Build multi-arch
+     run: |
+       rustup target add x86_64-unknown-linux-musl
+       rustup target add aarch64-unknown-linux-musl
+       cargo build --release --target x86_64-unknown-linux-musl -p miroir-proxy
+       cargo build --release --target aarch64-unknown-linux-musl -p miroir-proxy
+   ```
+
+2. **Docker Image**:
+   - Build manifest list spanning `linux/amd64` + `linux/arm64`
+   - Publish to `ghcr.io/jedarden/miroir:<version>`
+
+3. **Testing**:
+   - Phase 9 CI: Add arm64 test runs on arm64 runners
+
+## Triggers for Promotion
+
+Promote this bead to in-progress when:
+- [ ] Fleet includes ARM nodes (Hetzner Ampere, AWS Graviton, GCP Tau T2A, Rackspace Spot)
+- [ ] Concrete deployment requirement emerges
+- [ ] CI infrastructure for arm64 is available/justified
+
+## References
+
+- Plan §15 Open Problem #6
+- https://github.com/jedarden/miroir/issues/[TBD]