fix(llvm): avoid stale function handles after pre-autodiff optimization

[martinjrobins]

Summary

• refresh key LLVM function handles by name after pre_autodiff_optimisation() before invoking Enzyme gradient generation
• return explicit errors when expected functions are missing after optimization, rather than passing potentially stale handles into Enzyme
• harden CI cache key to include build-relevant file hashes and add a restore-prefix key

Why

CI showed deterministic Enzyme/LLVM segfaults in attribute handling. One likely failure mode is stale FunctionValue handles after aggressive pre-autodiff pass pipelines.

Validation

• cargo fmt
• local LLVM-enabled build/test path previously verified in workspace
• full run in the new worktree was blocked by submodule network restrictions in this environment