Fix slice lowering to pass explicit end bounds to OP_SUBSTR

[michaelsutton]

Fixes a compiler bug in ExprKind::Slice lowering where slice(start, end) was compiled as OP_SUBSTR(start, end - start) instead of OP_SUBSTR(start, end). The incorrect lowering caused invalid ranges and wrong slice lengths in runtime contracts (notably slice+concat reconstruction flows).

The PR includes:

• Compiler fix in compiler.rs
• Runtime regression test for explicit slice bounds in compiler_tests.rs
• Focused slice reconstruction/equality runtime test in compiler_tests.rs
• Updated runtime expectations for affected cashc examples in cashc_valid_examples_tests.rs