[codex] Fix section context delivery

[sukar0972]

Summary

• make section context injection depend on the turn-start message, not the total user-message count in the current projection
• fall back to the live section project context when a section thread is missing its snapshot
• add focused coverage for delayed first-turn handling and missing-snapshot fallback

Root Cause

Section context delivery used a fragile current-message-count check. If the projection already contained a later user message when the first turn-start event was processed, the first message could be treated as non-initial and sent without section context.

Validation

• vp check
• vp run typecheck