Summary

• Imported function bodies are compiled into the same WASM module via flattening (no WASM linking needed)
• ModuleCall nodes desugared to flat FnCall in wasm.py since imported functions exist locally
• Private helpers called by imported public functions are compiled automatically
• Guard rail updated: only truly undefined functions error; imported functions pass through
• vera compile and vera run now work with multi-module programs
• modules.vera example compiles and runs end-to-end

Test plan

• 951 tests pass (943 + 8 new cross-module codegen tests)
• mypy clean
• All 14 examples pass vera check and vera verify
• All validation scripts pass (README, spec, version sync)
• vera compile examples/modules.vera produces valid WASM
• vera run examples/modules.vera --fn abs_max -- -3 -5 returns 3
• vera run examples/modules.vera --fn clamp -- 10 1 5 returns 5

Closes #50
Closes part of #14

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