feat: scienting-math/1 sources and derived/math.jsonl; lode references; stress fixes

Math content may say its source is in scienting-math/1's canonical TeX (form,
in no hash); derived/math.jsonl holds papermath's tree, symbols and issues,
and the validator now checks derived files. A symbol annotation names its
symbol by key; lode claim references use lode's tag and pin syntax. The zip
reader refuses entries zipfile cannot read, and checkpoint lines end in LF only.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017R7sgDNvUGWY2zFPCq2JH6

schemas/node.schema.json

@@ -329,13 +329,28 @@
                   "source": {
                     "type": "string",
                     "minLength": 1
+                  },
+                  "form": {
+                    "$ref": "common.schema.json#/$defs/mathForm"
                   }
                 },
                 "required": [
                   "notation",
                   "source"
                 ],
-                "additionalProperties": false
+                "additionalProperties": false,
+                "if": {
+                  "required": [
+                    "form"
+                  ]
+                },
+                "then": {
+                  "properties": {
+                    "notation": {
+                      "const": "tex"
+                    }
+                  }
+                }
               }
             }
           }