fix: honor a Miss returned from the go() callback instead of wrapping it in a Hit

[andrewshell]

Summary

go() honored a returned Hit but wrapped every other return value — including a Miss — in a new Hit. A callback returning new Cacheism.Miss(...) to signal failure on an empty cache therefore produced a Hit whose .data was a Miss-shaped object (isHit === true while data.isMiss === true).

This adds a result instanceof Miss branch so a returned Miss is used as-is, symmetric with the existing returned-Hit handling:

if (result instanceof Hit) {
  response = result;
} else if (result instanceof Miss) {   // NEW
  response = result;
} else {
  response = new Hit(name, result);
}

The Miss is now persisted as a real Miss (preserving consecutiveErrors across the store round-trip) instead of a junk Hit.

Design note

A returned Miss is authoritative — it is not routed through the cacheOnFail/preferCache fallback to a stale cached Hit. That fallback stays exclusive to thrown errors. In short: a returned value is authoritative, while a throw delegates to the status policy.

Tests

Three new cases in test/memory.test.cjs (mirroring the existing "returns a Hit instance" block):

• returned Miss used directly on an empty cache
• returned Miss persisted, consecutiveErrors preserved through the store round-trip
• under cacheOnFail, a returned Miss overwrites a cached Hit (pins the authoritative-vs-fallback decision)

Full suite: 82 pass, 0 fail; typecheck clean.

Docs

README gains a "Callback return value" subsection documenting return-vs-throw semantics and the new Cacheism.Miss(existing.cacheName, …) guidance, plus a clarifying note on the "Cache on Fail" status.

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