Resolve FFI externals during type checking

[cknitt]

Why

Parsetree is the AST that PPXs see, and we want to freeze a version of it as "v1" at some point (#8624). Before that, it should only describe source code.

Externals didn't. After the frontend processed the FFI attributes of an external, it replaced the primitive string with the computed FFI spec (Prim_ffi, Prim_inline_const). As a result:

• parsetree.ml depended on External_ffi_types, the compiler's internal FFI representation. That type would become part of any frozen AST, and of a standalone syntax package we'd like to publish on opam.
• The AST type had cases a PPX can never see, because PPXs run before the frontend. The v0 bridge needed an error branch for them.

What changes

pval_prim is now just the string from the source, with its location (string loc option). For @send external join: … = "join" it is "join"; the @send stays an attribute.

The FFI attributes are now interpreted during type checking. Typedecl calls Primitive.resolve_external, which the frontend registers, since that code lives above compiler/ml. The type checker ends up with the same types and primitive descriptions as before, so the generated JS is unchanged.

The frontend still processes each external once, for two reasons:

• errors and warnings stay where they were;
• it needs to know whether the external uses a relative @module path, which stops other modules from inlining it.

It now leaves the declaration as written, though. The type checker's second pass runs with warnings off, so nothing is reported twice.

Smaller changes that follow from this:

• @inline constants used to be encoded as Prim_inline_const. They are now external x: T = "#rescript-inline" with the @inline(<literal>) attribute kept.
• The unused-attribute and leftover-json checks now look at the processed form of each external rather than the declaration as written.
• The outcome printer (hover, interface output) gets its own small type for processed primitives, Outcometree.out_primitive.
• Typedtree.val_prim is a string as well; gentype now reads the external's name from the type checker's primitive description.
• The AST and CMT magic numbers are bumped.

Tests

• New ast-mapping fixture with several kinds of externals, all printing back unchanged after the v0 round trip.
• New ounit test for an external's string and attributes surviving the v0 round trip.
• New super_errors fixtures for an unused attribute on an external's argument and a json literal that an external doesn't consume, both of which are now checked on the processed form.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 85.16129% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.83%. Comparing base (1cc1da7) to head (242e4d3).

Files with missing lines	Patch %	Lines
compiler/frontend/ast_external_mk.ml	78.94%	4 Missing ⚠️
compiler/ml/oprint.ml	0.00%	3 Missing ⚠️
compiler/ml/primitive.ml	40.00%	3 Missing ⚠️
compiler/frontend/ast_external.ml	93.33%	2 Missing ⚠️
compiler/frontend/bs_ast_invariant.ml	90.00%	2 Missing ⚠️
compiler/frontend/bs_builtin_ppx.ml	88.88%	2 Missing ⚠️
compiler/syntax/src/res_outcome_printer.ml	33.33%	2 Missing ⚠️
compiler/ml/printast.ml	0.00%	1 Missing ⚠️
compiler/ml/printtyped.ml	0.00%	1 Missing ⚠️
compiler/syntax/src/res_printer.ml	50.00%	1 Missing ⚠️
... and 2 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8728      +/-   ##
==========================================
+ Coverage   79.79%   79.83%   +0.04%     
==========================================
  Files         464      464              
  Lines       63134    63135       +1     
==========================================
+ Hits        50377    50406      +29     
+ Misses      12757    12729      -28     

Files with missing lines	Coverage Δ
compiler/core/lam_compile_external_call.ml	92.90% <ø> (ø)
compiler/frontend/ast_attributes.ml	94.59% <100.00%> (+2.70%)	⬆️
compiler/frontend/ast_exp_handle_external.ml	82.50% <100.00%> (ø)
compiler/frontend/ast_external_process.ml	77.81% <100.00%> (+1.65%)	⬆️
compiler/frontend/ppx_entry.ml	91.66% <100.00%> (+1.66%)	⬆️
compiler/gentype/translation.ml	92.85% <100.00%> (+1.42%)	⬆️
compiler/ml/ast_iterator.ml	94.55% <100.00%> (+0.01%)	⬆️
compiler/ml/ast_mapper.ml	90.15% <100.00%> (+0.02%)	⬆️
compiler/ml/ast_mapper_from0.ml	72.46% <100.00%> (+0.03%)	⬆️
compiler/ml/ast_mapper_to0.ml	72.23% <100.00%> (+0.28%)	⬆️
... and 18 more

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commit: 242e4d3

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