Improve missing footprint function errors

[CodeForFee]

Summary

• replace the generic invalid footprint-function error with an explicit “Function not found for footprinter …” message
• include the missing function name and original footprint string when available
• add regression coverage for invalid string footprints and builder calls with no selected function

Closes #46

Verification

• bun test tests/fp-string-error.test.ts tests/vssop.test.ts
• bun test
• bun run build
• git diff --check

AI-assisted with Codex; I reviewed the diff and local verification before submission.