Improve refcount for unique continuations

[TimWhiting]

Fixes #854, and addresses a few todos.
@daanx

[Copilot]

Pull request overview

This PR refines continuation handling and reference counting for control handlers, specifically optimizing behavior when continuations are unique, and adds a regression test around refcounts. It addresses the leaked or stuck reference counts seen with complex effects (e.g., async) by improving how composed continuations and finalization prompts are managed.

Changes:

• Optimize kcompose in lib/std/core/inline/hnd.c to detect unique continuation blocks and avoid unnecessary dup/drop operations, with tailored handling when yielding.
• Rework the protect/protect2 logic in lib/std/core/hnd.kk to track finalization state via a new protect-state type and a new unsafe-st2 helper, improving correctness for resumptions and finalization under effects.
• Add test/cgen/simple-refcount.kk (and .kk.out) to validate simple refcount behavior around a custom ctl effect and resumption pattern.

Reviewed changes

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

File	Description
lib/std/core/inline/hnd.c	Adds uniqueness-aware path in kcompose to optimize composed continuation invocation and yielding.
lib/std/core/hnd.kk	Introduces protect-state and unsafe-st2, and rewrites protect / protect2 / protect-prompt to correctly manage finalization and resumptions with effects.
test/cgen/simple-refcount.kk / .kk.out	New cgen test verifying refcount evolution for a boxed value across effectful resumptions.

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