feat(wave3): PR3-wave-2 predictor reader migrations — runner + dead-constant + sector_map

[cipher813]

Completes the predictor-repo side of Wave-3 PR3 reader migration (ROADMAP L1401). The deployed Lambda chokepoint shipped 2026-05-19 via #181 (regime/features._read_parquet_close); this PR closes the analysis-tool sites the L1401 PR3-wave-2 plan flagged + the dead module-level constant.

What's in this PR

1. Dead-constant removal — config.PRICE_CACHE_KEY

git grep PRICE_CACHE_KEY returned zero references outside the constant itself. Deleting it prevents a future from config import PRICE_CACHE_KEY that would bypass the Wave-3 read-prefix chain. A regression-pin test (test_config_no_longer_exports_price_cache_key_constant) locks the deletion.

2. analysis/triple_barrier_cutover_runner._load_prices_from_s3

Per-ticker parquet lookup now iterates a per-callsite read-prefix chain (reference/ → predictor/). Inlines the same shape #181 used in regime/features._read_parquet_close rather than importing it — keeps analysis tools decoupled from Lambda-deployed modules.

3. analysis/triple_barrier_cutover_runner._load_sector_map_from_s3

Prepends reference/price_cache/sector_map.json to the existing fallback list (data/sector_map.json → predictor/price_cache/sector_map.json). sector_map.json now lives at three keys post alpha-engine-data#272's write-both gap fix; this reader prefers the new home but cascades through both legacy locations for the soak.

Tests

New file tests/test_triple_barrier_cutover_runner_wave3_readers.py — 7 tests:

• Price-cache 3-test discipline: prefers-new / falls-back-legacy / absent-in-both
• sector_map 3-test discipline: prefers-reference / cascades-through-data-then-predictor / returns-empty-on-total-miss
• PRICE_CACHE_KEY deletion guard

Suite: 1113 → 1120 green (+7).

Discovery surfaced during the audit

scripts/momentum_ic_study_21d.py is named in the L1401 PR3-wave-2 plan as a Wave-3 reader site. git status + git check-ignore confirm it is untracked / never committed — a local-only operator analysis script (not gitignored, just not added). I migrated it in the main repo working copy so it survives PR4 cutover, but did NOT add it to git in this PR. Brian's call whether to git-track it later; the migration is reversible either way.

The file's download_price_cache() now iterates the read-prefix chain with the same dedupe-by-basename logic as alpha-engine-data#273's list_price_cache_keys, and the hardcoded SECTOR_MAP_KEY constant was dropped (it was unused after the read path through the synced cache directory remained correct).

PR4 cutover

Post-soak (earliest ~2026-05-26), PR4 drops the legacy entry from both fallback tuples here in one edit; the sector_map fallback collapses to reference/price_cache/sector_map.json only (or a 2-key chain pending data/ retirement timing).

Composes with

• alpha-engine-predictor feat(wave3): try reference/price_cache/ first in regime/features._read_parquet_close (ROADMAP L1401) #181 (the deployed Lambda chokepoint this mirrors)
• alpha-engine-data #272 (sector_map.json 3-key write-both that this reads from)
• alpha-engine-data #273 (sibling PR3-wave-2 — data-repo invasive sites)
• [[reference-wave3-read-new-first-with-legacy-fallback-pattern]]

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