Fix compiler crash for options of transitive dependency types

[fhammerschmidt]

Constructing Some(Facade.v) or passing Facade.v to an optional argument currently crashes with Fatal error: exception Not_found when its type is declared in a transitive dependency whose interface is absent from the consumer's search path.

Match directly on the type-declaration lookup so a missing declaration takes the conservative option-wrapping path. Known representations retain their existing behavior, and only Not_found from the lookup is caught.

Add a cross-module build regression that compiles the consumer with only the facade's include path, then executes the generated JavaScript. It covers variants, records, aliases, abstract types, unboxed types, optional arguments, and preservation of Some(undefined) versus None. Add focused unit coverage for unavailable declarations and known integer types.

Fixes #8687.

Validation:

• The new build regression fails with Not_found before the fix and passes afterward.
• make test passes, including formatting, 330 compiler unit tests, 558 runtime tests, build fixtures, and 741 documentation examples.

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.03%. Comparing base (95bdbd1) to head (3898ccd).
⚠️ Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
compiler/ml/typeopt.ml	66.66%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8719      +/-   ##
==========================================
+ Coverage   78.95%   79.03%   +0.08%     
==========================================
  Files         474      475       +1     
  Lines       64231    64180      -51     
==========================================
+ Hits        50711    50724      +13     
+ Misses      13520    13456      -64     

Files with missing lines	Coverage Δ
tests/ounit_tests/ounit_tests_main.ml	100.00% <ø> (ø)
tests/ounit_tests/ounit_typeopt_tests.ml	100.00% <100.00%> (ø)
compiler/ml/typeopt.ml	82.69% <66.66%> (+3.44%)	⬆️

... and 8 files with indirect coverage changes

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commit: 3898ccd

[cknitt]

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