Fix Windows extension feature detection.

[samuel-williams-shopify]

Summary

• add Windows to the main CI test matrix
• use have_func with an actual rb_tracearg_instruction_sequence call expression so mkmf rejects missing header declarations
• avoid an unused tracer variable warning in the raw hook build path
• add an ## Unreleased release note for the Windows native extension build fix
• fall back to the TracePoint object API when Ruby exposes the symbol but not the C header declaration

Notes

The Windows failure happened because probing the bare symbol can succeed when the symbol is linkable even if ruby/debug.h does not declare it. The call-expression form fails on implicit declarations and still defines HAVE_RB_TRACEARG_INSTRUCTION_SEQUENCE when supported.

ext/extconf.h is generated by create_header; it is already ignored by .gitignore and remains untracked.

Validation

• bundle exec bake test
• bundle exec rubocop