Refresh P2E runtime before workloads

[daiyongyuan]

Summary

• Name P2E --diff bitstream build directories <chip>-diff-<timestamp> while retaining <chip>-<timestamp> for normal builds and honoring an explicit output directory.
• Before --runworkload, let Cargo rebuild the selected case's P2E runtime when software sources change, then install the updated executable and, for DiffTest, libriscv.so.
• Build and run DiffTest with the same <chip>-p2e-diff target directory so vdbg loads the newly built libbebop_p2e.so. Cargo skips compilation when its inputs are unchanged. A build failure stops before FPGA simulation.

The stale-runtime trigger was a W0-only DPI change: rerunning LeNet against an existing bitstream reused a bebop-p2e executable built before the Rust change and loaded a DPI library from the ordinary target directory.

Verification

• Python AST syntax check and git diff --check for the runworkload handler.
• CI Build and GitGuardian checks passed.
• A full P2E workload rerun was not available in this session.

This change is required by DangoSys/buckyball#125 alongside DangoSys/bebop#24.