docs(specs): add M175–M181 Rust daemon port family; park M173/M174

Seven-milestone family porting agentsfleetd from Zig to Rust behind the
frozen /v1/runners wire seam: workspace scaffold, substrate, runner
control plane, tenant/workspace surface, admin/operator surface, signed
ingress + cron + connectors, and cutover with rehearsed rollback.
Layout follows the bun canon (rustd/src/afd_*, dir == crate). Specs
passed the SPEC TEMPLATE GATE, an adversarial review pass, and a Codex
review pass; all findings addressed in-place.

M173_001 and M174_001 (Zig test-surface work) move to done/ as DEFERRED
with Indy-acked quotes; reactivation is gated on M181 readiness.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>