feat(checker): cph-schema — 4 kind JSON Schemas + validation (WU-3)

Declarative JSON Schema per kind (segment/example/lemma/sop), embedded as real
.json artifacts (ADR-0006) and validated by a small hand-rolled interpreter
(properties/required/type/enum/additionalProperties + the `x-cph-content`
marker) — no jsonschema-crate churn for a one-optional-string scalar surface.
`x-cph-content: true` marks content fields (not in element.toml; require sibling
<field>.typ); everything else is an element.toml scalar. `validate(desc)` is
self-contained: UnknownKind for unknown kinds, MissingContentFile (path named in
message) for absent required .typ, SchemaViolation for bad scalars; lemma proof
optional ⇒ no diagnostic when absent. 8 tests green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/cph-schema/schemas/example.json

@@ -0,0 +1,13 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "cph:kind/example",
+  "title": "example",
+  "type": "object",
+  "properties": {
+    "problem": { "type": "string", "x-cph-content": true },
+    "solution": { "type": "string", "x-cph-content": true },
+    "source": { "type": "string" }
+  },
+  "required": ["problem", "solution"],
+  "additionalProperties": false
+}

crates/cph-schema/schemas/lemma.json

@@ -0,0 +1,12 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "cph:kind/lemma",
+  "title": "lemma",
+  "type": "object",
+  "properties": {
+    "stmt": { "type": "string", "x-cph-content": true },
+    "proof": { "type": "string", "x-cph-content": true }
+  },
+  "required": ["stmt"],
+  "additionalProperties": false
+}

crates/cph-schema/schemas/segment.json

@@ -0,0 +1,11 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "cph:kind/segment",
+  "title": "segment",
+  "type": "object",
+  "properties": {
+    "textbook": { "type": "string", "x-cph-content": true }
+  },
+  "required": ["textbook"],
+  "additionalProperties": false
+}

crates/cph-schema/schemas/sop.json

@@ -0,0 +1,11 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "cph:kind/sop",
+  "title": "sop",
+  "type": "object",
+  "properties": {
+    "sop": { "type": "string", "x-cph-content": true }
+  },
+  "required": ["sop"],
+  "additionalProperties": false
+}