docs: record Smart DCA P3 evidence boundary

docs/evidence/nasdaq_sp500_smart_dca/README.md

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+# Nasdaq/S&P 500 Smart DCA — P3 evidence boundary
+
+This directory is reserved for the unified P3 evidence package for
+`nasdaq_sp500_smart_dca`.
+
+The current research notes and tests are not promoted to a P3 evidence package
+yet. The existing sweep is a price-only proxy and does not include a committed,
+hash-pinned input manifest, point-in-time validation, or a complete cost/turnover
+ledger. Those omissions must be resolved before the lifecycle matrix can mark
+P3 as verified.
+
+## Current status
+
+- lifecycle stage: `P3`
+- status: `DEFERRED`
+- authority: research only; `no_order=true`
+- source notes: `docs/research/nasdaq_sp500_smart_dca.md`
+- follow-up matrix: `docs/research/nasdaq_sp500_price_proxy_matrix_2026-06-19.md`
+
+## Required artifacts before registration
+
+1. Hash-pinned QQQ/SPY proxy input manifest.
+2. Frozen configuration snapshot and code revision.
+3. Benchmark and cost-model records.
+4. Reproducible trial ledger and locked holdout result.
+5. Validated evidence package consumed by `gate_evidence_package.py`.
+
+No file in this directory asserts performance, shadow, paper, live, or capital
+authority. It is intentionally a visible placeholder so the missing evidence
+cannot be mistaken for an untracked implementation gap.