CANP-WP-0002 T01: input defaults, static and derived

Closes the gap that made optional inputs unusable: rendering rule 5.1(4)
made any unresolved placeholder an error while inputs had no `default`, so
an input marked `required: false` and referenced from the template failed
every render in which the caller omitted it — including the spec's own
section 4 example.

Section 10 already carried the derivation mechanism (`requirement: generate`,
resolution deliberately undefined), so a derived default needed a binding
rather than a new concept: the input's default names a declared prompt
dependency.

Spec:
- 5.1 rewritten as "Resolution and rendering". Resolution may be
  non-deterministic and must report what it derived; rendering is
  deterministic and must not derive. A tool that handles only supplied
  values and static defaults is stated to be conforming.
- 6.1 (new) covers both declaration forms. Reference form is preferred, with
  the reason stated — an inline prompt is anonymous, so unversioned,
  unprovenanced and un-evaluable — and validators should warn when a
  published package derives inline.
- A derived default may declare a static fallback `value`. Without one the
  input stays unresolved, which is an error; derivation never silently
  yields empty content.
- 4, 10, 18 (rules 11-13), 19 (two new MUST NOTs), 21 updated accordingly.

Reference CLI:
- New `resolve` verb reporting the origin of every value.
- `Resolution` dataclass and `resolve_inputs`; `resolve_values` kept as a
  wrapper so existing callers are unaffected.
- `render` refuses with a specific error naming underivable inputs rather
  than substituting empty text.
- Tests 3 -> 11. Example package lifecycle re-verified end to end.

INTENT.md is unchanged: splitting resolve from render preserves success
criterion 4 (deterministic rendering) as written.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Bjefh8NUiEiahN4JLwoSKM

Assistant: claude-code
Assistant-Model: opus
Assistant-Process: 388925@bnt-lap001
Assistant-Session: 3507023f-e0fd-4a1e-9d90-a0d4217d1502

reference/tests/test_canned_prompts.py

@@ -51,3 +51,172 @@ def test_undeclared_placeholder_fails(package: Path) -> None:
     (package / "prompt.md").write_text("{{ missing }}\n", encoding="utf-8")
     with pytest.raises(cp.CannedPromptError, match="undeclared placeholders"):
         cp.validate_package(package)
+
+
+def write_pkg(pkg: Path, manifest: str, template: str = "{{ greeting }}\n") -> Path:
+    pkg.mkdir(exist_ok=True)
+    (pkg / "prompt.yaml").write_text(manifest, encoding="utf-8")
+    (pkg / "prompt.md").write_text(template, encoding="utf-8")
+    return pkg
+
+
+BASE = """\
+format: canned-prompt/v0.1
+id: demo/defaults
+name: Defaults
+version: 1.0.0
+summary: Exercise input defaults.
+template: prompt.md
+"""
+
+
+def test_static_default_fills_optional_input(tmp_path: Path) -> None:
+    pkg = write_pkg(
+        tmp_path / "p",
+        BASE
+        + """\
+inputs:
+  - name: greeting
+    required: false
+    default: "hello there"
+""",
+    )
+    manifest = cp.validate_package(pkg)
+    resolution = cp.resolve_inputs(manifest, {})
+    assert resolution.values["greeting"] == "hello there"
+    assert resolution.origins["greeting"] == "default"
+    assert cp.render_template("{{ greeting }}", resolution.values) == "hello there"
+
+
+def test_supplied_value_overrides_static_default(tmp_path: Path) -> None:
+    pkg = write_pkg(
+        tmp_path / "p",
+        BASE
+        + """\
+inputs:
+  - name: greeting
+    required: false
+    default: "hello there"
+""",
+    )
+    manifest = cp.validate_package(pkg)
+    resolution = cp.resolve_inputs(manifest, {"greeting": "hi"})
+    assert resolution.values["greeting"] == "hi"
+    assert resolution.origins["greeting"] == "supplied"
+
+
+def test_derived_default_uses_static_fallback(tmp_path: Path) -> None:
+    pkg = write_pkg(
+        tmp_path / "p",
+        BASE
+        + """\
+dependencies:
+  prompts:
+    - id: context/greeting
+      version: 1.0.0
+      requirement: generate
+inputs:
+  - name: greeting
+    required: false
+    default:
+      derive: context/greeting
+      value: "(none)"
+""",
+    )
+    manifest = cp.validate_package(pkg)
+    resolution = cp.resolve_inputs(manifest, {})
+    assert resolution.values["greeting"] == "(none)"
+    assert resolution.origins["greeting"] == "fallback (not derived)"
+    assert resolution.underivable == []
+
+
+def test_derived_default_without_fallback_is_underivable(tmp_path: Path) -> None:
+    pkg = write_pkg(
+        tmp_path / "p",
+        BASE
+        + """\
+dependencies:
+  prompts:
+    - id: context/greeting
+      version: 1.0.0
+      requirement: generate
+inputs:
+  - name: greeting
+    required: false
+    default:
+      derive: context/greeting
+""",
+    )
+    manifest = cp.validate_package(pkg)
+    resolution = cp.resolve_inputs(manifest, {})
+    assert resolution.underivable == ["greeting"]
+    assert "greeting" not in resolution.values
+    with pytest.raises(cp.CannedPromptError, match="unresolved placeholder"):
+        cp.render_template("{{ greeting }}", resolution.values)
+
+
+def test_inline_derive_is_valid(tmp_path: Path) -> None:
+    pkg = write_pkg(
+        tmp_path / "p",
+        BASE
+        + """\
+inputs:
+  - name: greeting
+    required: false
+    default:
+      derive:
+        prompt: Produce a greeting suited to the audience.
+      value: "(none)"
+""",
+    )
+    manifest = cp.validate_package(pkg)
+    assert cp.resolve_inputs(manifest, {}).values["greeting"] == "(none)"
+
+
+def test_default_with_required_true_fails(tmp_path: Path) -> None:
+    pkg = write_pkg(
+        tmp_path / "p",
+        BASE
+        + """\
+inputs:
+  - name: greeting
+    required: true
+    default: "hello"
+""",
+    )
+    with pytest.raises(cp.CannedPromptError, match="required: true"):
+        cp.validate_package(pkg)
+
+
+def test_derive_reference_must_be_declared(tmp_path: Path) -> None:
+    pkg = write_pkg(
+        tmp_path / "p",
+        BASE
+        + """\
+inputs:
+  - name: greeting
+    required: false
+    default:
+      derive: context/greeting
+""",
+    )
+    with pytest.raises(cp.CannedPromptError, match="not declared in dependencies"):
+        cp.validate_package(pkg)
+
+
+def test_inline_derive_cannot_also_reference(tmp_path: Path) -> None:
+    pkg = write_pkg(
+        tmp_path / "p",
+        BASE
+        + """\
+inputs:
+  - name: greeting
+    required: false
+    default:
+      derive:
+        prompt: Produce a greeting.
+        id: context/greeting
+""",
+    )
+    with pytest.raises(cp.CannedPromptError, match="cannot also reference"):
+        cp.validate_package(pkg)