docs: mark smolmachines-specific PRDs + research as superseded

Add a "Superseded (2026-07-11)" banner and flip Status: Active →
Superseded on the 8 smolmachines-specific docs (7 PRDs + the VM-backend
research). Bodies are left intact as a historical record; the banner
points at the removal commit and the landscape doc so a reader isn't
misled into thinking the backend still exists.

Incidental mentions in other PRDs (git-gate, cred-proxy, print-parity,
etc.) are left as-is.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WBMWTEtQdJ4W5UrWuLHCck

docs/prds/0042-smolmachines-parity-tests.md

@@ -1,6 +1,8 @@
 # PRD 0042: smolmachines Cross-Backend Parity Tests
 
-- **Status:** Active
+> **Superseded (2026-07-11).** The smolmachines backend was removed — Linux now uses the Firecracker backend, macOS uses macos-container. Kept as a historical record; see the removal commit `c07ebca` and `docs/research/landscape-containerized-claude.md`.
+
+- **Status:** Superseded (2026-07-11) — was Active
 - **Author:** didericis-codex
 - **Created:** 2026-06-02
 - **Issue:** #139