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/annotation.schema.json

@@ -111,15 +111,13 @@
               "const": "SpecificResource"
             },
             "source": {
-              "type": "string",
-              "minLength": 1
+              "$ref": "common.schema.json#/$defs/claimRef"
             },
             "kind": {
               "const": "claim-link"
             },
             "pin": {
-              "type": "string",
-              "minLength": 1
+              "$ref": "common.schema.json#/$defs/claimPin"
             },
             "role": {
               "enum": [
@@ -144,17 +142,21 @@
               "const": "SpecificResource"
             },
             "source": {
-              "type": "string",
-              "minLength": 1
+              "$ref": "common.schema.json#/$defs/claimRef"
             },
             "kind": {
               "const": "symbol"
             },
             "pin": {
+              "$ref": "common.schema.json#/$defs/claimPin"
+            },
+            "key": {
+              "description": "The symbol's scienting-math/1 key, which the symbol claim's notation has too (spec/09 §9.4, spec/18 §18.4).",
               "type": "string",
               "minLength": 1
             },
             "symbol": {
+              "description": "The symbol as the span writes it.",
               "type": "string",
               "minLength": 1
             }
@@ -162,7 +164,8 @@
           "required": [
             "type",
             "kind",
-            "source"
+            "source",
+            "key"
           ],
           "additionalProperties": false
         },