docs: add PRD 0042

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

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+# PRD 0042: smolmachines Cross-Backend Parity Tests
+
+- **Status:** Draft
+- **Author:** didericis-codex
+- **Created:** 2026-06-02
+- **Issue:** #139
+
+## Summary
+
+Add tests that prove secrets, forwarded env, resume, and remediation behave
+equivalently across Docker and smolmachines backends. The fixes in PRDs
+0038–0040 are unverifiable without this coverage.
+
+## Problem
+
+The existing unit suite is broad but backend-specific. There are no tests that
+run the same scenario against both Docker and smolmachines and assert the
+outcomes match. A regression in one backend goes undetected until a live run,
+and PRDs 0038–0040 can each pass their own unit tests while the backends still
+diverge at the integration boundary.
+
+## Goals / Success Criteria
+
+- A parity test suite that covers at least:
+  - Secret env injection: `?prompt` and `${HOST_VAR}` entries produce the same
+    guest env on both backends.
+  - Forwarded env: literal manifest env values reach the guest on both backends.
+  - Resume: a preserved bottle state dir round-trips correctly on both backends
+    (relies on PRD 0040 metadata).
+  - Remediation: capability-block approval routes to the correct backend handler
+    (relies on PRD 0039 dispatch).
+- Each scenario is parameterised so a failure names the backend that regressed.
+- Tests run without a live VM or Docker daemon (mock or stub backends).
+
+## Non-goals
+
+- No end-to-end agent execution tests.
+- No performance or load tests.
+- No changes to production code (test-only PRD).
+
+## Scope
+
+In scope:
+
+- New test file(s) under `tests/unit/` for parity scenarios.
+- Stub or mock implementations of smolmachines and Docker backends as needed.
+
+Out of scope:
+
+- Changes to `bot_bottle/` production code.
+- CI infrastructure changes beyond adding the new test file to the discover
+  invocation.
+
+## Dependencies
+
+- PRD 0038 should land before the env parity tests are finalised.
+- PRDs 0039 and 0040 should land before the remediation and resume scenarios
+  are finalised; stubs can be written speculatively beforehand.
+
+## Design
+
+Parameterise each scenario over a list of backend factory functions. Each
+factory returns a bottle instance wired to a stub subprocess layer. The test
+body is backend-agnostic: it calls the same public API, captures the same
+observable output, and asserts equality.
+
+For env scenarios, capture the argv or env-file content passed to the guest
+and compare against resolved manifest values. For resume, write metadata with
+one backend class and read it back to verify correct selection. For remediation,
+assert dispatch selects the per-backend handler.
+
+## Testing Strategy
+
+Run as part of the standard unit discover:
+
+- `python3 -m unittest discover -s tests/unit`
+
+Or directly:
+
+- `python3 -m unittest tests.unit.test_backend_parity`
+
+## Open Questions
+
+- Should parity tests live under `tests/unit/` (mock-based) or
+  `tests/integration/` (live infra)? Mock-based is preferred to keep CI simple.