create --type change
change: `spt update` runs its adapter legs concurrently, one thread per adapter, after the core leg
Milestone body item (releases#331): "all adapter updates performed during `spt update` become asynchronous (one thread per adapter), still performed after the spt-core update."

Today `cmd_adapter_update` (cli.rs ~22270) walks the selected adapters in one loop on one thread, so N adapters cost the sum of N fetches + post-steps. Ask: after the core leg (unchanged order: core first, REQ-UPDATE-DEFAULT-COMPOSITE), fan the adapters leg out one thread per adapter; each thread buffers its own output and the parent prints each adapter's block when that adapter finishes (no interleaved lines), then the unchanged `ADAPTER_UPDATE_SUMMARY:` lines and the 0/3/1 exit (REQ-UPDATE-REFUSAL-EXIT-DISTINCT). Post-steps (`[update.post]`) run inside their adapter's thread. The floor basis stays the staged core (REQ-ADAPTER-FLOOR-VS-STAGED-CORE). Same fan-out serves the background updater's adapters leg.

Gate: unit on the per-adapter output isolation; int: three mock adapters with sleeps complete in ~max not ~sum, summary/exit unchanged from the serial run.