feat: warn when recompute is cheaper than cache retrieval

[raminjafary]

Adds an opt-in warnWhenSlower option to CacheOptions. When enabled, ocache
times cache retrieval (a hit get) against recompute (the resolver on a miss/
refresh) and emits a one-time console.warn when recomputing the value is
cheaper than reading it back from the cache — i.e. caching is adding net
overhead for that cache.

• Off by default; only tracks when the option is set, so no cost for existing users.
• Measured directly around the resolver and storage read (no function wrapping),
  so integrity hashing is unaffected. Handles request dedup and SWR refreshes.
• Works for both defineCachedFunction and defineCachedHandler.

Addresses nitrojs/nitro#2559/nitrojs/nitro#4401 (Nitro enables it in dev via import.meta.dev).

Usage:

const getUser = defineCachedFunction(fetchUser, { warnWhenSlower: true })

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Reviewing files that changed from the base of the PR and between b0339a2 and 6f69741.

📒 Files selected for processing (4)

• README.md
• src/cache.ts
• src/types.ts
• test/index.test.ts

👮 Files not reviewed due to content moderation or server errors (4)

• src/types.ts
• test/index.test.ts
• README.md
• src/cache.ts

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