v0.1.369 — negative constant tuple index t[-k] (PMAT-670)

Correctness (PMAT-670): a negative constant tuple index t[-1] now resolves to the field access t.(arity-1) at compile time (Python from-the-end) instead of emitting list-style .len()/[] indexing (E0599 — Rust tuples have neither). Works for heterogeneous tuples too. An out-of-range literal and a runtime-variable tuple index now reject cleanly (a heterogeneous fixed-arity tuple can't be runtime-indexed). Found by the HUNT-V5 differential hunt; verified vs python3. 427 e2e fixtures.