fix: clarify footprint function errors

[vahapogut]

Fixes #46

Summary

• Add one shared diagnostic for missing/invalid footprint functions.
• Include the missing function name in both string parser and fluent API errors.
• Add regression coverage for string, fluent, and empty footprinter error paths.

Verification

• bun test tests/fp-string-error.test.ts tests/dip.test.ts
• bun x biome check src/footprinter.ts tests/fp-string-error.test.ts
• bun run build
• git diff --check