fix(validate): reject ptr<CallbackName> as unsupported pointer-to-callback shape

[detail-app]

Detail bug report: View on Detail

Bug

A function parameter declared param cb type=ptr<Sink>, where Sink is a declared callback signature, passed validation even though it describes a pointer to a function pointer — the same shape already rejected when spelled ptr<fn<Sink>>. Callback names were registered into the shared types set used to validate pointer pointees, so the "unknown pointee" check accepted them. But callbacks are lowered as function-pointer typedefs (typedef void (*Sink)(...)), not struct tags, so the C backend rendered struct Sink * for a struct Sink that was never declared — producing a header that compiles with a warning, fails under -Werror, and is unusable at C call sites without casts.

Struct fields using ptr<Sink> were rejected only by accident of validation ordering (fields validated before callback names were registered), while function parameters validated afterward passed.

Closes #5

Fix

Two changes in Validator.java:

• type() method: Added a require(!signatures.contains(n.name()), ...) check in the Pointer(Named) branch so a pointer whose Named pointee is a callback signature is rejected with "pointers to callbacks are not supported; use fn<Name>", matching the existing ptr<fn<Sink>> rejection.
• validate() method: Moved callback-name registration before struct-field validation so both struct fields and function parameters reject ptr<CallbackName> for the same stated reason, rather than struct fields rejecting it only by accident of ordering with a less helpful "unknown pointee" message.

Testing

• Added two negative test cases to callbacksAndIsolateThreadsAreValidated: ptr<Sink> as a function parameter and ptr<Sink> as a struct field — both must throw IllegalArgumentException.
• All 152 JVM tests pass (including the 77-case parameterized negative-input suite and golden output comparison).
• mvn -B verify passes including the JaCoCo coverage gate (≥85% line, ≥70% branch).
• E2e integration test (tests/e2e.py) passes: generated C header compiles under cc -std=c11 -Wall -Werror, Rust compiles, and FFM round-trip check passes.
• Manual reproduction confirmed ptr<Sink> throws the expected message in both contexts, while fn<Sink>, ptr<Handle> (opaque), and ptr<ptr<u8>> (nested) continue to work correctly.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.82%. Comparing base (c7d278e) to head (b4ab1ae).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff              @@
##               main       #7      +/-   ##
============================================
+ Coverage     93.77%   93.82%   +0.05%     
- Complexity      656      658       +2     
============================================
  Files            18       18              
  Lines          1895     1895              
  Branches        391      392       +1     
============================================
+ Hits           1777     1778       +1     
  Misses           37       37              
+ Partials         81       80       -1     

Flag	Coverage Δ
jvm	93.82% <100.00%> (+0.05%)	⬆️

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