docs: curate 0.3.1 release notes

planning/releases/0.3.1.md

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+# compose2pod 0.3.1 — the `image` mount type
+
+A focused follow-up to 0.3.0: the long-form `volumes` `type: image` mount, which
+closes the last long-form volume form Podman can express but compose2pod did not
+yet parse.
+
+## Feature
+
+- **`type: image` volume mounts.** A long-form entry `{type: image, source,
+  target, read_only, image: {subpath}}` compiles to `podman run --mount
+  type=image,source=<ref>,target=<path>[,subpath=<abs>]`, mounting an image's
+  rootfs into the container. Measured against `docker compose config` v5.1.2 and
+  Podman 6.0.1:
+  - `source` (the image reference) is **required** — Podman refuses a sourceless
+    image mount, so compose2pod does too (a rule-two narrowing; Docker allows it
+    optional at config).
+  - `read_only` is accepted but **inert** — an image mount is always read-only,
+    and Podman rejects both a `readonly` and an `rw` option on it, so no `ro` is
+    emitted (matching what Docker does with the field).
+  - `image: {subpath}` must be an **absolute** path — Podman requires it for an
+    image mount (a relative `subpath` is rejected at run time), unlike a `volume`
+    `subpath`, which may be relative. A `${VAR}` is carved out.
+
+## Docs
+
+- The README's supported-subset section, stale since before the 0.3.0 parity
+  work, now leads with the measured parity property (compose2pod refuses every
+  document `docker compose config` refuses, verified by the differential
+  conformance harness) and reflects the full 0.3.0 subset.
+
+## Downstream
+
+- `type: image` compose files that previously raised now compile. Nothing else
+  changes — the short form, the other long-form types, and every existing key
+  behave identically.
+- With `image` landed, the honest subset is complete for its scope: every
+  remaining refusal of a form Docker accepts is either a permanent rule-two limit
+  (Podman cannot form the flag) or a genuinely niche gap (a Windows drive-letter
+  volume source on a Linux-targeting tool), each measured and documented in
+  `architecture/supported-subset.md`, `planning/deferred.md`, and
+  `planning/decisions/`.
+
+## Internals
+
+- Tests at 100% line coverage (enforced); `ruff select=ALL`, `ty`, `eof-fixer`
+  clean; the conformance harness (real Docker CLI) and integration harness (real
+  Podman) both green — the integration suite mounts an image's rootfs on real
+  Podman, and the final review re-measured every emitted `--mount` option against
+  Podman 6.0.1, catching (and closing) a relative-`subpath` value Podman rejects
+  at run time.