feat(hpm): add descriptions to HPM events (#157)

Carry an event description together with each HPM name and value, preserving the metadata while events pass through module boundaries. Add the small HPMDocDump utility that writes the four event sets to build/hpm.csv when elaboration requests a dump.

Keep the legacy tuple-based HasPerfEvents API unchanged and add a separate HasPerfEventInfos trait for description-aware events. HPerfCounter and HPerfMonitor use the new trait, while getPerfEvents remains available so branches that have not migrated still compile.