Extract Belt into a separate @rescript/belt package

[cknitt]

Summary

Extract Belt from @rescript/runtime into a separately installed, source-distributed @rescript/belt package.

• Move Belt sources and generated artifacts into the new package
• Remove compiler behavior that depends specifically on Belt
• Lower array and list spreads to runtime primitives instead of Belt.*.concatMany
• Keep Belt available in the playground and API documentation
• Add package installation coverage for npm and pnpm
• Separate Belt tests from the compiler/runtime test suite
• Update affected fixtures, dependencies, tooling, and generated output

Breaking change

Belt is no longer bundled with the ReScript runtime. Projects that use Belt must install it:

npm install @rescript/belt

They must also list it in rescript.json:

{
  "dependencies": ["@rescript/belt"]
}

Existing Belt.* APIs remain available after adding the dependency.

Rewatch note

The compiler repository builds the Belt source dependency through test projects with different package output settings. Rewatch can currently reuse stale dependency artifacts when switching between those settings.

This PR explicitly cleans and rebuilds Belt where required as a temporary workaround. The underlying invalidation problem is tracked separately in #8539 and is not fixed by this PR.

Testing

• make test
• make test-syntax
• make test-gentype
• make test-analysis

Closes #8050

Related to #8539

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[codecov]

Codecov Report

❌ Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.79%. Comparing base (3f1843d) to head (0750bf7).

Files with missing lines	Patch %	Lines
compiler/core/outcome_printer_ns.ml	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8554      +/-   ##
==========================================
+ Coverage   75.54%   75.79%   +0.24%     
==========================================
  Files         476      476              
  Lines       62710    62680      -30     
==========================================
+ Hits        47374    47506     +132     
+ Misses      15336    15174     -162     

Files with missing lines	Coverage Δ
analysis/src/semantic_tokens.ml	89.14% <100.00%> (ø)
compiler/frontend/bs_builtin_ppx.ml	90.55% <ø> (ø)
compiler/syntax/src/res_comments_table.ml	89.39% <ø> (ø)
compiler/syntax/src/res_core.ml	91.58% <ø> (+0.05%)	⬆️
compiler/syntax/src/res_diagnostics.ml	77.77% <ø> (ø)
compiler/syntax/src/res_grammar.ml	42.05% <ø> (ø)
compiler/syntax/src/res_parsetree_viewer.ml	91.10% <ø> (ø)
compiler/syntax/src/res_printer.ml	93.15% <100.00%> (ø)
compiler/core/outcome_printer_ns.ml	0.00% <0.00%> (ø)

... and 30 files with indirect coverage changes

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[pkg-pr-new]

Open in StackBlitz

rescript

npm i https://pkg.pr.new/rescript-lang/rescript@8554

@rescript/belt

npm i https://pkg.pr.new/rescript-lang/rescript/@rescript/belt@8554

@rescript/darwin-arm64

npm i https://pkg.pr.new/rescript-lang/rescript/@rescript/darwin-arm64@8554

@rescript/darwin-x64

npm i https://pkg.pr.new/rescript-lang/rescript/@rescript/darwin-x64@8554

@rescript/linux-arm64

npm i https://pkg.pr.new/rescript-lang/rescript/@rescript/linux-arm64@8554

@rescript/linux-x64

npm i https://pkg.pr.new/rescript-lang/rescript/@rescript/linux-x64@8554

@rescript/runtime

npm i https://pkg.pr.new/rescript-lang/rescript/@rescript/runtime@8554

@rescript/win32-x64

npm i https://pkg.pr.new/rescript-lang/rescript/@rescript/win32-x64@8554

commit: 0750bf7

[github-actions]

Developer playground preview: https://rescript-lang.github.io/rescript/dev-playground/?version=pr-8554

[cknitt]

@codex review

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Reviewed commit: 60b8ecb78b

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[cknitt]

@codex review

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Codex Review: Didn't find any major issues. Keep them coming!

Reviewed commit: a026aa0324

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