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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[TimWhiting]

I ran the benchmarks from here: https://github.com/effect-handlers/effect-handlers-bench:

There are slight improvements in:

• nqueens: ~6%
• generator/tree_explore: ~10.5%
• resume_nontail: ~24%

Everything else is roughly equal and within variation from multiple runs I did.

Old:

command,mean,stddev,median,user,system,min,max
countdown/main 200000000,1.2216245841200002,0.013097457960270271,1.2170492882200001,1.2134481199999998,0.0047678,1.20872678772,1.25088287172
fibonacci_recursive/main 42,0.92421362602,0.009763783503512343,0.92031885122,0.9161674200000001,0.0042211,0.9134147467200001,0.9404233717200001
product_early/main 100000,1.3867592175199999,0.0304284327448526,1.37119780922,1.3794159199999998,0.0049787,1.35771262172,1.44621662172
iterator/main 40000000,0.165431218095,0.007390034056625232,0.16279237172,0.15923437,0.0022573625000000003,0.15822895472,0.18062316372
nqueens/main 12,1.1569119590199999,0.017909991041971343,1.15707007972,1.1479616199999998,0.004852799999999999,1.13427649672,1.1834659547200002
generator/main 25,5.7108088426200005,0.07435528029232083,5.71386903872,5.68219942,0.014105600000000001,5.55519399672,5.82056137172
tree_explore/main 16,0.19325579672,0.001078928400741546,0.19339541372,0.19085892000000002,0.0014779666666666666,0.19167212172,0.19593866272
triples/main 300 300,0.8743085257200001,0.0068642368323112105,0.8727008712200002,0.8700542199999999,0.0025719000000000002,0.8651514967200001,0.8840237467200001
parsing_dollars/main 20000,1.82072437592,0.004804321725366059,1.8187204552200003,1.81371392,0.004718000000000001,1.8146368297200002,1.83010133072
resume_nontail/main 20000,1.91535725102,0.007202490545974237,1.9139477467200001,1.9084864199999998,0.0044865,1.90591020472,1.92681853872
handler_sieve/main 60000,2.70993834242,0.03705139933226507,2.71244768422,2.6999992199999996,0.006494,2.66178566272,2.7925084547199996

New:

command,mean,stddev,median,user,system,min,max
countdown/main 200000000,1.2312289023800003,0.0061499745787337535,1.22874679838,1.2249821199999997,0.00403104,1.2245549018800002,1.2417574448800002
fibonacci_recursive/main 42,0.95989003598,0.011211520612998738,0.9594421103799999,0.95143202,0.00473814,0.94594815288,0.97980498588
product_early/main 100000,1.4064860191800004,0.017392783664945167,1.40584848638,1.3975502199999998,0.00568144,1.38559302688,1.4490656938800002
iterator/main 40000000,0.15521384551157894,0.0008896818988581859,0.15497898588,0.15272484631578948,0.0013665505263157894,0.15430290288,0.15742473588
nqueens/main 12,1.08314209418,0.006838441225641589,1.08064050638,1.0773020199999999,0.0036328399999999996,1.07480056988,1.09845361088
generator/main 25,5.11096174408,0.0395463510170471,5.10337152738,5.086660519999999,0.01659444,5.06295198588,5.17069323588
tree_explore/main 16,0.1726601770564706,0.0016844145028007576,0.17251623588,0.16973302588235295,0.0016064576470588236,0.17055256888,0.17756581888
triples/main 300 300,0.75250363178,0.00578785742406358,0.75097075688,0.7477755199999999,0.00307514,0.74416244388,0.76534673588
parsing_dollars/main 20000,1.8222293193800003,0.004684792777568356,1.8222978818800002,1.8132483199999996,0.0060284399999999995,1.8152147358800002,1.8296958198800002
resume_nontail/main 20000,1.45199178978,0.006093713514123297,1.4520157978800001,1.44437892,0.00481684,1.4406204858800002,1.4619268608800002
handler_sieve/main 60000,2.72472791088,0.044018776042209384,2.71512896488,2.71188212,0.00880684,2.68808611088,2.83538506988