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

spec/02-model.md

@@ -84,6 +84,7 @@ The stop words are `a an the of on in for to and or at by with from as into onto
 **The identity** is the work's first identifier among `doi`, `arxiv`, `isbn`, `pmid`, `pmcid` and `openalex` in `same_as`, in that order, written `<kind>:<value>` with a DOI's ASCII letters lower-cased.
 A work with none has the identity `meta:<title words>|<year>|<family>` from the same folded parts.
 `url:` and `lode:` identifiers name copies and claims, not the work, and never enter the identity.
+A `lode:` identifier is a claim reference as a layer writes one, `lode:<tag>` or `lode:<host>/<tag>` ([18.4](18-layers.md#184-bodies)).
 
 **The hash part** is the leading bits of SHA-256 of `scienting-key/1:` followed by the identity, in UTF-8, written in lowercase Crockford base32, five bits a character: four characters, or, when another work in the store already holds that key, six, eight and so on until the key is free.
 A key is minted once, when the work enters a store, and never recomputed: a corrected title or a year revised later leaves it alone, while the hash of an identifier still shows which work the key names.