[P2][Plan 5] SF-027 — Remove duplicate Screenshot Plan from Markdown export

[Ankit6149]

Confirmed problem

The Markdown generator appends the Screenshot Plan section twice, producing noisy and misleading export output.

Scope

• Trace every Markdown composition path and establish one canonical section order.
• Generate Screenshot Plan exactly once when relevant and omit it when no plan exists.
• Use the shared export projector introduced in Plan 1 rather than route/UI string concatenation.
• Add section-level uniqueness assertions.

Acceptance criteria

• Screenshot Plan appears at most once in Markdown.
• No other campaign section is duplicated through nested package composition.
• Edited channel drafts still appear exactly once.
• Empty optional sections are omitted cleanly.
• Markdown remains deterministic across save/reopen/export.

Required verification

• Golden Markdown snapshots with and without screenshot plans.
• Duplicate-heading assertion across the full document.
• Regression test using an edited multi-channel campaign.