Add CacheConfig: typed cache-tag/lifetime functor over Cache

[reebalazs]

Adds CacheConfig (GreenfinityNext.CacheConfig) — a functor wrapping the Cache 'use cache' tag + lifetime bindings with typed variants, so each app supplies its own cache-tag type while sharing one implementation. (Lifts the per-app helper that lived in njc-2026's Content_Cache.)

API

module type CacheConfig = {
  type tag
  type life
  let tagToString: tag => string
  let lifeToString: life => string
}

module DefaultsLife = {                       // @as("seconds") Seconds … @as("max") Max
  type life = Seconds | Minutes | Hours | Days | Max
  let lifeToString = (life: life) => (life :> string)
}

module Make = (Config: CacheConfig) => {
  // re-exports tag/life/tagToString/lifeToString + cacheTag/cacheLife/updateTag/revalidateTag
}

Why tagToString in the config (not a coercion inside the functor)

(tag :> string) is compile-checked and only works on a concrete @as-string variant; an abstract functor type (Config.tag) "is not a subtype of string", so the coercion has to happen where the concrete type is known — the app's tagToString. There's no built-in "coercible-to-string" constraint and no type classes, so passing the conversion is the idiomatic stand-in. DefaultsLife carries the common lifetime enum so apps include it.

Usage

The config must be a named module (the functor result mentions Config.tag/Config.life, so an inline struct gives "parameter cannot be eliminated"):

module MyCacheConfigBase = {
  include DefaultsLife
  type tag = | @as("public-content") PublicContent
  let tagToString = (tag: tag) => (tag :> string)
}
module MyCacheConfig = Make(MyCacheConfigBase)
MyCacheConfig.cacheTag(MyCacheConfigBase.PublicContent)