fix(ir): derive internal function return types from declarations

[N0xMare]

Summary

Fixes #49 — Internal (non-pub) functions had their return type hardcoded to u256 in IR lowering, regardless of their actual declaration. Functions returning bool, addr, void, or tuples all got wrong IR type annotations.

The Bug

crates/ir/src/to_egglog/function.rs in lower_internal_function_body():

let out_ty = EvmType::Base(EvmBaseType::UIntT(256)); // TODO: derive from return type

This meant every internal function call site assumed a u256 return, which could silently corrupt values for functions returning bool, addr, or other types.

The Fix

Thread the declared return type through the call chain:

1. ContractFunction type — extended from 3-tuple to 4-tuple, adding Vec<TypeSig> for returns
2. emit_call — now accepts and forwards returns to lower_internal_function_body
3. lower_internal_function_body — replaces hardcoded UIntT(256) with self.returns_to_type(returns)

This mirrors what lower_function() already does correctly for standalone free functions — returns_to_type() handles all cases: 0 returns → UnitT, 1 return → lower_type_sig, multiple → TupleT.

Files Changed

File	Change
crates/ir/src/to_egglog/mod.rs	ContractFunction type + population site
crates/ir/src/to_egglog/function.rs	lower_internal_function_body signature + return type derivation
crates/ir/src/to_egglog/calls.rs	emit_call signature + all call sites

Tests Added

3 e2e tests verifying internal function return types work correctly at runtime:

1. Bool return — internal check_positive(x) -> (bool) called from pub fn; verifies both true and false cases
2. U256 return (regression) — internal double(x) -> (u256) still works correctly
3. Void/storage — internal function with storage side effects works correctly

All 656+ workspace tests pass.

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[brockelmore]

lgtm