docs: add 0.1.8 release notes

planning/releases/0.1.8.md

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+# compose2pod 0.1.8 — interpolation moves to run time
+
+This release reverses 0.1.7: Compose `${VAR}` interpolation now happens when
+the *generated script* runs, not when compose2pod generates it. compose2pod
+emits a script that runs later — typically in CI, where the real secrets and
+config live — so resolving against compose2pod's own environment at generation
+time baked any variable that was unset then to an empty string. References are
+now emitted as live POSIX-shell fragments that the runtime shell expands.
+
+## Change
+
+- **`${VAR}` is interpolated at script-run time, not generation time.**
+  `to_shell()` (new `compose2pod/shell.py`) re-encodes every compose-derived
+  value (`environment`, `image`, `command`, `volumes`, `tmpfs`, `env_file`,
+  healthcheck `test`) into a double-quoted shell fragment whose variable
+  references stay live, so the generated script's own shell resolves them
+  against its runtime environment. All the 0.1.7 forms still work — `$VAR`,
+  `${VAR}`, `${VAR:-default}`, `${VAR-default}`, `${VAR:?msg}`, `${VAR?msg}`,
+  `${VAR:+alt}`, `${VAR+alt}`, `$$` — mapping onto identical POSIX `sh`
+  parameter expansion. A bare `$VAR`/`${VAR}` is emitted as `${VAR-}` so an
+  unset variable expands to empty under the script's `set -eu` instead of
+  aborting; `${VAR:?msg}` now fails the script at run time (not at generation).
+  Command substitution and other shell metacharacters in literal text are
+  escaped, so nothing but a variable reference is ever live. The CLI prints one
+  informational note listing the variables the script references at run time.
+
+## Fix
+
+- **`environment` null value is host passthrough.** A null mapping value
+  (`KEY:`) now emits a bare `-e KEY` — passing `KEY` through from the host,
+  identical to the list form `- KEY` — instead of the literal `-e KEY=None`.
+- **Malformed braced references are rejected.** A braced reference whose text
+  after the name is not a valid operator (e.g. `${FOO!bar}`) now raises a clear
+  `UnsupportedComposeError` instead of silently dropping the trailing text.
+
+## Packaging
+
+- Public API: the generation-time `interpolate` function is **removed**;
+  `to_shell` is exported in its place. `to_shell(value)` returns a
+  runtime-expandable shell fragment, not a resolved value.
+
+## Why
+
+The 0.1.7 model resolved `${VAR}` against `os.environ` at generation time. For
+compose2pod's actual use — generate a script now, run it later in CI — that was
+the wrong moment: the variables are supposed to be unset at generation and set
+at run time, so 0.1.7 baked them to empty and emitted a spurious "variable not
+set" warning. Deferring to the runtime shell resolves them where they exist.
+
+## Downstream
+
+- **Compose files:** set `${VAR}` values in the *runtime* environment of the
+  generated script (e.g. your CI job), not in the environment that runs
+  compose2pod. An unset bare `${VAR}` now expands to empty at run time; write
+  `${VAR:?message}` where you want a missing variable to fail the run.
+- **API consumers:** `compose2pod.interpolate` is gone. Use
+  `compose2pod.to_shell(value)`, which emits a runtime-expandable shell fragment
+  rather than resolving the value in-process.
+
+## Internals
+
+- `architecture/supported-subset.md` rewritten to describe run-time expansion,
+  the `${VAR-}`/`$$`/`${VAR:?}` semantics, `env_file` interpolation, the
+  reference note, null-value passthrough, and malformed-reference rejection.
+- 123 tests at 100% line coverage (enforced); `ruff select=ALL`, `ty`, and
+  `eof-fixer` clean. `test_shell.py` executes emitted fragments under real
+  `sh -euc` to prove run-time expansion, the `${VAR:?}` abort, and
+  command-substitution inertness.