fix(chat): render agent turns as chronological segments with scroll-follow

[ABB65]

Problem

During a long agent turn the user sees only the Thinking spinner; when the turn ends, every intermediate narration ("Şimdi 8 makaleyi oluşturuyorum…") lands as one glued, present-tense wall of text. Verified on staging (conversation `8ad2bba3…`):

• The client appended all text deltas of a multi-iteration turn into one bubble (no separators) and dumped every tool card below it — chronology (text → tools → text → tools) was destroyed.
• Auto-scroll fired only on new messages, never during streaming, so live narration accumulated off-screen.
• On reload, tool cards vanished entirely: history hydration read the legacy `tool_calls` column (always null since the trace migration) instead of `content_blocks`.

Changes

• useChat: `ChatMessage` now carries ordered `segments` (`text` | `tool`) built live from the SSE stream. New `messageText`/`hasVisibleContent` helpers; `streamTick` counter drives scroll-follow.
• ChatPanel: assistant messages render segment-by-segment in real order; pin-to-bottom scroll-follow (releases when the user scrolls up, re-pins near the bottom).
• ChatMarkdown atom: extracted throttled markdown pipeline from ChatBubble — finished segments freeze; only the trailing streaming segment pays the reparse cost.
• Engine (additive): new `tool_input` SSE event at `tool_use_end` (card shows its input during the whole pending window) and `input` on `tool_result` as the guaranteed carrier. The ee conversation-api response is unchanged — it projects `{id, name, result}` explicitly.
• History hydration prefers `content_blocks` over legacy `tool_calls`; the literal `[tool calls]` placeholder can no longer render.

Known/accepted: markdown split across a tool call renders as two independent documents (models don't straddle fences across tool calls in practice).

Follow-up PR (2/2) will make intermediate iterations visible in persisted history and harden the graceful-close summary.

Test

• `pnpm test` — 130 files / 1075 tests green
• `pnpm vitest run --project e2e tests/e2e/chat-stream.e2e.test.ts` — DOM-order + input assertions
• `pnpm lint` + `pnpm typecheck` clean

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