docs(certification): broadcast TV training content for A1, S1, B3

[bokelley]

Summary

Closes #2047. Adds broadcast TV training content across three certification modules, completing the instruction path that IACET Element 5 requires before assessment. Depends on broadcast TV protocol PR #2046 (merged 2026-04-14).

• A1 (foundations): Appends "broadcast impression data matures over days after air (C3/C7 windows, not real-time), which you'll work with concretely in S1" to the 20-channels bullet — closes the prerequisite gap flagged in Training content: broadcast TV concepts and exercises #2047 so learners arrive at S1 knowing why broadcast measures differently.
• S1 (media buy mastery): Adds broadcast delivery interpretation to "What you'll demonstrate" (C3/C7 window maturation, incomplete data as by-design not underdelivery); extends lab exercise 8 from billing-only to "Broadcast billing and delivery" with a get_media_buy_delivery interpretation task.
• B3 (publisher track): Renames ### Exercise → ### Exercises and adds a seller-perspective broadcast delivery scenario: given measurement_windows: [{window_id: "c3"}, {window_id: "c7"}], describe available data two days post-flight and how to represent it accurately.
• Migration 423: Registers SuccessCriterion IDs for the two new demonstrations (s1_ex1_sc_broadcast_delivery_windows, b3_ex1_sc_broadcast_delivery_seller_communication) so Addie enforces them in the rubric and recertification triggers for existing credential holders.

Pre-PR review fixes applied

• Corrected window → window_id in B3 exercise example (schema field name)
• Clarified C7 phrasing: "DVR accumulation doesn't close until seven days post-air, and vendor processing adds further delay" (previously conflated accumulation end with data availability)
• Changed A1 from "panel schedule" to "matures over days after air" (panel/diary applies to radio; broadcast TV uses return-path/set-top data)

Test plan

• Verify three MDX pages render without MDX errors in the docs preview
• Confirm window_id field in B3 exercise matches the schema at static/schemas/source/core/measurement-window.json
• Confirm migration 423 runs clean against a local DB (_append_criterion calls for S1 and B3)
• Check that A1, S1, B3 module links in the "What's next" section of publisher.mdx still resolve

https://claude.ai/code/session_016dJ4nMXCbcLEmCsCNrBK6d

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