Remove Primitive_option.toUndefined; use valFromOption for optional ffi args

[cknitt]

Summary

This PR removes Primitive_option.toUndefined and switches optional FFI argument lowering to use Primitive_option.valFromOption instead.

valFromOption already has the right runtime behavior here:

• None stays undefined
• Some(v) becomes v
• nested options still unwrap by exactly one outer layer

As a result, the extra toUndefined helper is redundant for this code path and can be deleted.

Changes

• change compiler/core/js_of_lam_option.ml so the non-duplicable optional-argument path emits Primitive_option.valFromOption(...) instead of Primitive_option.toUndefined(...)
• remove Primitive_option.toUndefined from the runtime source and interface
• regenerate the JS runtime artifacts
• update the optional FFI generated test output accordingly

Example

Before:

return hey(Primitive_option.toUndefined(f(x)), 3);

After:

return hey(Primitive_option.valFromOption(f(x)), 3);

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commit: a13afcf