Prepare 12.3.1 maintenance release

[cknitt]

Summary

Prepare the conservative ReScript 12.3.1 maintenance release from maintenance/12.x.

The fixes were backported chronologically and adapted where necessary to avoid pulling newer infrastructure or broad refactors:

• Fix rewatch warning replay after early compile errors #8408: replay stored Rewatch warnings after an early compile error
• Fix formatting of trailing comments before equals #8444: format trailing comments before = correctly
• analysis: fix namespaced reference lookup #8455: fix namespaced reference lookup
• Fix build crash when compiler output is not valid UTF-8 #8482: avoid a Rewatch crash on invalid UTF-8 compiler output
• Preserve multibyte characters when wrapping long source lines #8520: preserve multibyte characters when wrapping compiler code frames
• Fix exponentiation precedence for compound exponents #8550: preserve compound exponent precedence
• Enforce function arity in type inclusion and coercion #8559: backport only the three missing function-arity type-system guards and focused regressions
• Record written arrow arity before external lowering #8563: backport only the independent bare labeled-arrow arity fix
• set compiler, runtime, platform package, and Rewatch versions to 12.3.1

Scope

This intentionally excludes larger or more invasive master changes, notably:

• Fix incorrect analysis report for optional function args #8321, the large reanalyze change
• Harden the Parsetree0 PPX bridge #8561, the combined PPX bridge and wire-format refactor
• the broader diagnostic-reporting portion of Enforce function arity in type inclusion and coercion #8559
• the phantom-argument parser/printer rewrite from Record written arrow arity before external lowering #8563

The goal is to fix concrete correctness and crash issues without increasing maintenance-release regression risk.

Validation

• make test
• make test-analysis
• make test-syntax
• make test-rewatch
• Rewatch Rust unit tests and formatting checks

All pass locally.

[pkg-pr-new]

Open in StackBlitz

rescript

npm i https://pkg.pr.new/rescript@8565

@rescript/darwin-arm64

npm i https://pkg.pr.new/@rescript/darwin-arm64@8565

@rescript/darwin-x64

npm i https://pkg.pr.new/@rescript/darwin-x64@8565

@rescript/linux-arm64

npm i https://pkg.pr.new/@rescript/linux-arm64@8565

@rescript/linux-x64

npm i https://pkg.pr.new/@rescript/linux-x64@8565

@rescript/runtime

npm i https://pkg.pr.new/@rescript/runtime@8565

@rescript/win32-x64

npm i https://pkg.pr.new/@rescript/win32-x64@8565

commit: a1ad1df