Restore Amb bracket state when first-of exits

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Bug

first-of escaped through call/cc before active amb-bracket and amb-bracket-range continuations could run their leave! callbacks. A successful first-result search therefore left caller-owned mutable state marked.

Fix

Add an internal escape-cleanup chain. Bracketed choices register their leave! callback while their continuation is active, and first-of unwinds that chain before returning its first solution.

Validation

• Added a regression covering nested list/range brackets and verifying state returns to zero.
• dotnet run --project IronKernel -- test lib/IronKernel.Amb/IronKernel.Amb.ikproj (20 checks passed)
• dotnet test IronKernel.sln -c Release (225 passed)
• Interactive REPL reproduction now returns the first solution (3) with bracket state restored to 0.

  

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Note

Low Risk
Scoped fix to IronKernel.Amb search semantics with a regression test; no auth, I/O, or host integration changes.

Overview
Fixes a bug where first-of returned the first solution via call/cc without running active amb-bracket / amb-bracket-range leave! hooks, leaving caller mutable state inconsistent.

Introduces an escape cleanup chain (parallel to sink) and with-bracket, which registers each bracket’s teardown while its continuation runs. first-of routes the first emitted solution through that chain before returning, and amb-bracket / amb-bracket-range delegate bracketing to with-bracket instead of inline enter/leave around k.

Adds a regression test that nested list/range brackets under first-of restore test counter state to zero after the first result (3).

^{Reviewed by Cursor Bugbot for commit acdb952. Bugbot is set up for automated code reviews on this repo. Configure here.}

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