Optimize type variable scope map cloning with CoW

[jakebailey]

This restores the copy-on-write in the original Strada code, which saves the creation of new maps. On the testrunner test suite, this saves about 720k allocations.

[Copilot]

Pull request overview

This PR restores a copy-on-write approach for NodeBuilderContext’s per-signature scope maps to reduce allocations when cloning scope state during type serialization.

Changes:

• Added core.CloneOrMakeMap utility to clone an existing map or allocate an empty one when nil.
• Updated cloneNodeBuilderContext to share inherited maps and defer cloning until mutation is required.
• Updated node builder mutation sites to clone-on-first-write using core.CloneOrMakeMap.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
internal/core/core.go	Adds CloneOrMakeMap helper to support clone-on-write map patterns.
internal/checker/nodebuilderscopes.go	Switches scope cloning from eager map cloning to CoW by resetting “created” flags while sharing maps.
internal/checker/nodebuilderimpl.go	Clones the shared maps on first mutation using core.CloneOrMakeMap instead of allocating fresh empty maps.