fix: add hazmat/export fields to ProductSchema source

[azaczek]

Why

#25 added these fields directly to schemas/product.json by hand, but never updated the Zod source that generate-schemas.ts uses to produce that file. Running npm run generate:json-schemas on current develop silently deletes all 7 fields, since nothing in CI regenerates and diffs the committed schema against the Zod source. Verified this by running the generator before making any change — it removed the fields cleanly, no errors.

How

Added the same 7 fields to ProductCoreSchema (all z.string(), optional via the existing .partial()), then ran npm run generate:json-schemas. Resulting schemas/product.json has the same field names/descriptions/types as #25 intended, minus some malformed "value": {} artifacts from the original hand-edit (not a valid JSON Schema keyword, inert either way) and one typo fix ("Freigth" → "Freight").

Testing

• npm run typecheck — clean
• npm run lint — 0 errors (pre-existing warnings only, none in touched files)
• npm test — 224/224 passing