perf: disable cacheImmutable by default

[chenjiahan]

Disable cacheImmutable by default to reduce the perf overhead of getAsset().

#38

[Copilot]

Pull request overview

Disables cacheImmutable by default so immutable assets no longer automatically receive Cache-Control: ..., immutable unless explicitly opted in, reducing overhead on the request path.

Changes:

• Change middleware cache header behavior to only apply immutable caching when cacheImmutable === true.
• Update docs and type definitions to reflect the new default (false when omitted).
• Adjust middleware tests to explicitly set cacheImmutable: true where the immutable behavior is expected.

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/middleware.js	Changes immutable cache decision logic to require an explicit cacheImmutable: true.
test/middleware.test.js	Updates tests to opt into immutable caching explicitly and updates expectations for the new default.
README.md	Documents the new cacheImmutable default and updated semantics.
src/index.js	Updates JSDoc option docs for cacheImmutable default.
types/index.d.ts	Updates type docs/comments for cacheImmutable default.

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