Track ReScript compiler version compatibility — keep emitted syntax + compile-check pin moving with ReScript releases

[jagguji]

Problem

The generator emits ReScript that's valid for a specific compiler version — ReScript 12 today. As ReScript releases 13, 14, … two things can drift and we have no explicit process to catch it:

1. Emitted syntax may need to change. We rely on version-sensitive features: untagged/tagged variants (@unboxed, @tag(…) + @as(…), ~24 + ~2 uses in emit.mjs), React.component<props> (relies on JSX v4 lowering, ~6 uses), React.componentWithProps (the Labeled-args components (no HTML-attrs base) never get a nameable props type — blocks wrapper/override pattern #155 wrapper pattern), @as field/variant naming (~13), JsxDOM.*, first-class bigint. A future ReScript could deprecate/rename any of these, and generated bindings would stop compiling for consumers on the new version.
2. Our compile gate is pinned and could silently lag. The golden compile check (test/sandbox) pins rescript ^12.0.0 + @rescript/react ^0.13.0, and CI names the job "Golden compile (ReScript 12)". Nothing verifies the output still compiles on a newer ReScript, so a break would only surface downstream (as it did for the blend beta line — caught only by consumer validation).

There's already mild drift in the wild: blend-rescript builds against rescript ^12.2.0 / @rescript/react ^0.15.0, while our sandbox pins ^12.0.0 / ^0.13.0.

What we should track / decide

• A stated "targets ReScript N" contract (README already says "ReScript 12" in ~5 places — make it the single source of truth, e.g. a RESCRIPT_TARGET constant or a README badge).
• An "on new ReScript release" checklist, e.g.:
  • bump test/sandbox (rescript, @rescript/react) to the new version, re-run npm run test:compile — all goldens must still compile.
  • update the CI job name + any README "ReScript 12" references.
  • note any emitted-syntax change in docs/TYPE_MAPPING.md (like the @tag/@unboxed/bigint rows already do).
  • regenerate blend-rescript against it as the downstream smoke.
• Consider a compile MATRIX — run the golden-compile job against both the current target and the next ReScript (e.g. 12 + 13-rc) so a break is caught in our CI, not a consumer's.
• A compatibility note — which @juspay/rescript-bindgen version emits for which ReScript range (a small table in README/CHANGELOG), so consumers know what to pair.

Concrete anchors

• test/sandbox/package.json — the compile-check ReScript pin (^12.0.0, @rescript/react ^0.13.0).
• .github/workflows/ci.yml — the "Golden compile (ReScript 12)" job.
• src/emit.mjs — the version-sensitive syntax the emitter produces.
• README.md (lines ~8/40/115/185/193) + docs/TYPE_MAPPING.md — where "ReScript 12" is stated.

Why now

Filed as a tracking issue off the v1.3.0 release — no change needed today (ReScript 12 is current and stable), but we want a deliberate process so a future ReScript bump is a checklist we run, not a surprise a consumer reports.