feat: add chunked variants of `parIterWithCancel`

Add `parIterWithCancelChunked` functions for CoreM, MetaM, TermElabM, and TacticM that support chunking jobs into groups to reduce task creation overhead.

The original `parIterWithCancel` functions remain unchanged for backward compatibility. The new chunked variants accept `maxTasks` and `minChunkSize` parameters to control parallelism.

This enables PRs that use `parIterWithCancel` (like parallel library search and rewrites) to benefit from chunking by switching to the new `parIterWithCancelChunked` function with `maxTasks := 128`.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>
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