Add protocol fuzz coverage and CI

[jeffreyaven]

Summary

• Add six native Go fuzz targets with deterministic regression corpora and protocol response checks.
• Add a pinned, read-only-permission fuzz workflow for pull requests and scheduled campaigns, plus contributor guidance and a make target.
• Raise the Go toolchain floor to 1.25.3 without upgrading dependencies; include narrowly scoped input-safety fixes found during fuzzing.

Validation

• go test ./... on Go 1.25.3.
• Six 30-minute fuzz campaigns passed.
• StackQL importer package tests and full build passed with a temporary local replacement.
• make fuzz smoke test passed through Git Bash.

GitHub Actions execution remains to be validated by CI.