Find live toplevel bindings in Js_shake with a worklist

[cknitt]

Stacked on #8772.

Js_shake decides which toplevel bindings of a module to keep. It repeated a full pass over all toplevel statements, recomputing the free variables of every initializer (the code had a TODO: add hashtbl for a cache), until no new binding became live. A dependency chain that runs backwards through the module needs one pass per link, which is quadratic.

The live set is the least set that contains the exports, the free variables of statements with side effects and the bindings whose initializer has side effects, and that contains the free variables of the initializer of every live binding. live_idents now computes the free variables of each initializer once and follows them with a worklist, which gives that same set in linear time.

Measurements

Same method as #8769, compared with the previous PR in the stack. All 574 files of tests/tests/src (not only the benchmark corpus) compile to byte-identical JS and diagnostics.

	CPU time	Allocated words
Stress: a chain of 1500 private functions where only the last one is exported, plus a dead chain of 300	549 → 290 ms (−47%)	−54%
57 real files	≈ noise	−0.08%

Modules whose live set settled in a single pass now pay a little for the dependency table (e.g. +1.9% allocation on a file with 1800 string literals), which is why real files come out about even.

🤖 Generated with Claude Code

[cknitt]

@codex review

[chatgpt-codex-connector]

Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-10T10:49:34.890557Z	50b538b	Manual request

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[chatgpt-codex-connector]

💡 Codex Review

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Reviewed commit: ffd5c80988

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[codecov]

Codecov Report

❌ Patch coverage is 95.65217% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.66%. Comparing base (477b2fc) to head (36d26f2).

Files with missing lines	Patch %	Lines
compiler/core/js_shake.ml	95.65%	1 Missing ⚠️

Additional details and impacted files

@@                   Coverage Diff                    @@
##           perf-external-shadow    #8773      +/-   ##
========================================================
- Coverage                 80.66%   80.66%   -0.01%     
========================================================
  Files                       463      463              
  Lines                     62758    62755       -3     
========================================================
- Hits                      50623    50619       -4     
- Misses                    12135    12136       +1     

Files with missing lines	Coverage Δ
compiler/core/js_shake.ml	86.11% <95.65%> (-3.64%)	⬇️

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[cknitt]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. 🎉

Reviewed commit: 50b538bbf8

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commit: 36d26f2