feat: add P2E bank DiffTest

[daiyongyuan]

Summary

• reuse the Verilator Bank DiffTest session for the P2E FPGA backend
• add P2E --diff, --golden-elf, and --golden-pk CLI support
• canonicalize uninitialized bank bytes without modifying BEMU storage and handle asynchronous P2E RTL callbacks
• run the BEMU golden model in the background and report bank_diff.ndjson failures
• clean up vdbg/CTB correctly on Ctrl-C and error exits

Validation

• cargo test -p bebop-bank-hash -p bebop-rtl-trace -p bebop-p2e -p bebop-syscall
• cargo test --no-default-features --features p2e --bin bebop
• cargo check --no-default-features --features p2e --bin bebop
• Pebble generated BEMU lib tests: 6 passed
• P2E LeNet hardware run: pass=631 mismatch=0 missing_rtl=0 unexpected_rtl=0
• diff and non-diff Ctrl-C exit with code 1, vdbg exitcode 0, no residual processes