Streaming responses via Anthropic SSE

[owainlewis]

`Provider.Complete` today returns once with the full response. SSE streaming would: render assistant text as it arrives, surface tool calls as soon as they're emitted (vs the whole turn completing first), and make long generations feel alive in the TUI.

Scope

• Add `Provider.Stream(ctx, req) (<-chan StreamEvent, error)` (or extend `Complete` with a stream callback).
• Bubble Tea: `textBlock` already exists; new partial-text update path that mutates the last block instead of appending.
• Anthropic client: switch to `stream: true` and parse `event: …` / `data: …` SSE frames.
• Cooperative cancel still works — closing the channel on ctx done aborts the stream cleanly.

Acceptance

• Sonnet 4.6 generation visibly streams in the chat TUI.
• Tool-use blocks still arrive correctly (Anthropic delimits them as content-block boundaries in streaming mode).
• `neo flow` headless still works (line-printer can stream or buffer — pick buffer for cleanest CI logs).
• Tests cover SSE parsing edge cases: split frames mid-buffer, comment lines, terminal `event: message_stop`.

[owainlewis]

Closing as a deliberate product decision: neo's TUI uses blocking + spinner rather than streaming token deltas. Streaming corrupts the status display and is intentionally out of scope. Reopen if that decision changes.