#118 serve: routing traces from the live server

Description

Two gaps kept the routing trace one-shot-only:

- drive_pool_multi, the verify-window dispatch the serve path uses for every decode call, had no trace hook; --dump-routing under --serve opened the file and wrote nothing. Records now go out per token in drive_pool's format (layer, k, ids, weights). The multi dispatch fuses the router weights into the kernel and does not surface them, so records carry unit weights: make_profile.py ranks pairs by routed frequency, which is the signal that matters; a one-shot run still records true weights.
- the trace sat in a stdio buffer until teardown, so a crash lost the whole session's data. It now flushes at every request's DONE.

Verified live: a serve-mode trace grows per request, parses cleanly in make_profile.py's format (all 48 layers, records per decode position), and captures real multi-session traffic for expert-profile building.