fix: accept mixed indices for buffer access

[nahime0]

Summary

• Allow buffer<T> reads and writes to accept Mixed indices produced by non-constant integer arithmetic.
• Narrow buffer read indices from Mixed to Int in EIR before backend bounds and address checks.
• Add end-to-end regression coverage and an Unreleased changelog entry.

Context

Checked integer addition, subtraction, and multiplication produce a boxed Mixed result so they can promote to float on
overflow. Buffer indexing still required a statically concrete Int, causing these indices to fail during type checking or EIR
backend lowering.

Buffer writes already performed the appropriate EIR coercion. This change makes reads follow the same target-neutral path while
allowing Mixed indices through the checker for both operations.

Fixes #497

Testing

• cargo build
• cargo test --test codegen_tests codegen::buffers:: — 15 passed
• cargo test --test error_tests buffer_ — 16 passed
• git diff --check