refactor(parser): one AssistantCallDetail per API response

Claude Code splits one response's blocks across transcript lines sharing
a message.id; buildTurn now folds them (last line's usage/timestamp — it
accompanies stop_reason — first line's prevTimestamp), so consumers see
one call per response. The daemon's response-grouping layer collapses to
direct lookups and chatSpans.ts goes away, its three survivors inlined
as module helpers in daemon.ts, their only consumer. Type-guards
(readTranscriptLine) replace the parser's remaining as-casts; dead
totalUsage() deleted.

Also a comment pass across the touched files: doc blocks cut to the
load-bearing why.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: rgao-coreweave