Fix parser crash on reserved pattern binders

[liufengyun]

Fix parser crashes on reserved words in value binder positions.

Summary

• reject an empty expression pattern with a source-located parser diagnostic
• cover section in both val and for binder positions
• cover the other reported block keywords (union, view, interface, and namespace)

Checklist

• Added parsing regression tests under tests/parsing/
• Docs updated if the change affects user-visible behavior — diagnostics only
• All commits are signed off (why?)

Security impact

No.

Compatibility impact

Reserved words used as pattern binders now produce a parser diagnostic instead of crashing. Valid source syntax and serialized formats are unchanged.

• Source compatibility: existing valid code continues to compile
• SAST compatibility
  • forward compatibility: new libraries can be used by old compiler
  • backward compatibility: old libraries can be used by new compiler
• Standard Library compatibility
  • forward compatibility: new code can work with old stdlib
  • backward compatibility: old code works with the new stdlib
• Runtime Library compatibility
  • forward compatibility: new code can work with old runtime
  • backward compatibility: old code works with the new runtime
• Build tool
  • Build spec compatibility: old projects continue to build
  • Joy package compatibility: old .joy files can be consumed by the new build tool

Verification

• bin/fuzz regress --parse tests/parsing — all 20 files pass without crashes
• direct parser replay of all six new cases reports Expect a pattern, found token ...
• git diff --check