[FEATURE]: Show per-part timestamps for thinking, tool calls, and tool output in the TUI

Describe the enhancement you want to request

The TUI has a /timestamps toggle (session.toggle.timestamps) that shows the message-level creation time under each message. There is currently no way to see when each individual block happened — thinking, tool invocation (e.g. bash command), and tool output — which makes it hard to tell whether a long-running agent is stalled in one step or just slow.

Requested: when timestamps are enabled, each part should render its own start timestamp (using part.time.start):

• Thinking block header: Thought: plan migration · 2.3s · 3:41 PM
• Tool invocation (inline): $ npm run build · 3:41 PM
• Tool block header (bash output, reads, etc.): # Running in packages/tui · 3:41 PM
• Message-level timestamp stays as-is

Parts already carry time.start/time.end in the protocol (packages/schema/src/v1/session.ts), so this is a pure TUI rendering change.

Why this would be useful

1. Detect stalls: see that a tool call has been running since 3:41 PM without output
2. Correlate agent activity with git commits / CI logs / deploys
3. Audit long sessions where multiple runs happen close together

Related issues

• #35348 (message/run start+completion timestamps; comments explicitly request per-step timestamps)
• #22144 (timestamp+duration on tool execution blocks — closed, no implementation landed)
• #42124 (timestamps on permission prompts)