docs(prediction): define event cohort architecture

[proerror77]

Change contract

Document the Polymarket event-root, symbol-by-horizon mission, shared Chainlink cohort, and settlement-versus-token-execution boundaries introduced by #143, without changing runtime behavior.

Out of scope

• Rust behavior, schemas, collectors, evaluators, or mission templates
• UP/DOWN token execution heads
• Full-update-tick collection
• Governed 15-minute or one-hour implementation
• ECS, deployment, Paper/Shadow, or live-trading changes

Dependency or merge order

Stacked on #143 (codex/polymarket-event-settlement-contract). Merge #143 first, retarget this PR to main, rerun current-head checks, then merge this documentation layer.

Focused validation

• git diff --check
• Diff against the base branch contains exactly four documentation/TODO files and 64 non-generated changed lines.
• Terminology matches fix(prediction): bind settlement splits to time cohorts #143 mission schema v2, binary manifest v3, and the governed 300-second settlement baseline.

Rollout / rollback impact

Documentation-only. No runtime or artifact behavior changes. Roll back by reverting this PR after #143; no data, collector, ECS, execution, or live state is mutated.

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Review details

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 86bf22e2-1ecb-4eb4-a603-c42a4d814a22

📥 Commits

Reviewing files that changed from the base of the PR and between ecbc49a and f8cff04.

📒 Files selected for processing (4)

• docs/architecture/PREDICTION_MARKETS.md
• rust_hft/prediction-markets/docs/ALPHA_FACTOR_SEARCH_CICD.md
• rust_hft/prediction-markets/docs/CRYPTO_LOB_ML_DEPLOY_CHECKLIST.md
• rust_hft/prediction-markets/tasks/todo.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/polymarket-event-cohort-docs

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