fix(llmapp): order streamed text via sealed control segments

[sqlrush]

Summary

Implements spec-1.21.1 stream ordering fix.

• routes authoritative assistant text through sealed ctrl FIFO segments
• keeps TokenStream as preview-only with PreviewTick + Model.previewNodes
• makes View read-only over committed scrollback + preview nodes
• removes old per-turn markAssistantBullet path and terminal-chunk mapToRender path
• syncs hook-local spec registry ordinal 50

Verification

• make gate
• go test ./internal/app/cli/llmapp
• go test ./internal/app/cli/...
• go test -race -count=10 ./internal/app/cli/llmapp -run TestModel_LoopAppendsToolBlocks

[sqlrush]

spec-1.21.1 implementation update — CI green

Implemented the R2.1 sealed-control architecture and fixed the transient CI security drift.

Implementation status

• Authoritative assistant text now travels through the same streamControlMsg FIFO as tool/result/finish events via SealedText.
• TokenStream is preview-only; View renders cached previewNodes and no longer drains or mutates scrollback.
• Tool-before/after assistant text now commits as separate assistant message nodes, restoring the post-tool ⏺ behavior without touching FROZEN spec-1.6.
• Removed the old terminal mapToRender drain path and the assistantBulletPending / markAssistantBullet mechanism.

Verification

• Local: make gate PASS.
• Local targeted: go test -race -count=10 ./internal/app/cli/llmapp -run TestModel_LoopAppendsToolBlocks PASS.
• PR CI: all required checks PASS on head e7fb459.
• Security drift: GO-2026-5037 / GO-2026-5039 added to the existing stdlib allowlist window; local make vuln-check PASS and CI security PASS.

Codex local review note

No new blocking finding from the implementation diff. Permanent transcript ordering is controlled by ctrl FIFO. Preview remains best-effort/transient by design and is cleared when sealed text commits, so producer run-ahead cannot pollute scrollback.