C1: cache-fill coordinator foundation in fork

[lspassos1]

Parent: #14
Registry prerequisite: #5

Problem

server/_shared/redis.ts is the shared cache primitive for server handlers. The distributed coordinator cannot rely on the old URL-encoded SET write path for larger JSON payloads, and it cannot safely use runRedisPipeline() for lock scripting because pipeline normalization assumes a simple key-at-index-1 command shape.

Scope

• add generated server-side cache-fill policy projection from registry/datasets.ts
• validate cacheFill invariants during registry:generate
• move setCachedJson() to a dedicated body-command Redis path
• add miss-only distributed coordination in server/_shared/redis.ts
• derive lock keys from the final prefixed Redis key
• use token-safe unlock from the start
• add structured outcome logging in the shared helper
• keep behavior unchanged when no generated policy exists or Redis coordination is unavailable

Out of scope

• expanding the allowlist beyond the first two shared keys
• migrating unrelated ad hoc locks in handler-specific code
• introducing a metrics backend beyond structured logs

Invariants

• mandatory cache recheck after lock acquisition
• followers poll the data key, not the lock key
• waitMs < leaseMs
• pollMinMs > 0
• pollMaxMs >= pollMinMs
• pollMaxMs < waitMs
• Redis lock errors degrade to legacy local singleflight behavior

Expected files

• registry/datasets.ts
• scripts/generate-dataset-registry.ts
• scripts/check-dataset-registry.mjs
• server/_shared/_generated/cache-fill-registry.ts
• server/_shared/redis.ts
• fake Redis / shared cache tests

Validation

• npm run registry:generate
• npm run registry:check
• node --test tests/redis-caching.test.mjs
• npm run typecheck
• npm run typecheck:api
• npm run test:data