docs/refactor: document data-layer contracts and dedupe token parsing (D1)

[StackOverFlow11]

Summary

Data Layer D1 — the first low-risk data-layer cleanup. Two things only:

1. a committed doc making the cache vs store boundary explicit;
2. removing the stale duplicated token parsing in TushareFeed and
   IndexConstituentsFeed by routing both onto the existing
   data/feed/secret.py::read_token (the four sibling feeds already use it).

Behavior-preserving: no data-fetch, cache, factor, alpha, portfolio, runtime,
or report-metric behavior changes.

Changed files (4)

• new docs/data/data_layer_contracts.md — states: data/cache is the
  reusable raw endpoint cache / source of truth; data/store/PanelStore is a
  per-run canonical panel artifact (not the raw lake); durable cache rows stay
  raw only; front_adjust / PIT index membership / PIT SW industry / ann_date
  financial alignment / raw price-limit checks / factors/alpha/portfolio/runtime
  math all remain downstream; data-update warms caches only; data-quality and
  concurrency are later phases (not claimed implemented).
• mod data/feed/tushare_feed.py — drop the duplicated module-level
  _lookup_dotted and the inline json/Path parsing; _read_token is now a
  one-line wrapper over read_token(self._secret_file, self._token_key).
• mod data/feed/index_feed.py — stop importing the private _lookup_dotted
  from tushare_feed.py; _client reads the token via read_token directly.
• mod tests/test_index_feed.py — +2 narrow tests exercising the index
  feed's real _client() token path with a fake tushare.pro_api (token sourced
  from a fake external config, never leaked; missing-key error stays readable).
  No test reads the real .config.json; no test hits the network.

_lookup_dotted is now fully removed repo-wide; all six tushare-backed feeds
(feed/index/flags/fina/intraday/covariates) source the token through
secret.read_token.

Verification (rebased onto current main)

• pytest -p no:cacheprovider → 579 passed (current main baseline 577 + 2 new
  index-feed tests; rose from Codex's earlier 540 only because the branch now sits
  on newer main, not because D1 changed).
• ruff check . → All checks passed!
• validate-config over all 17 config/*.yaml → all OK.
• run-phase0 --config config/example.yaml →
  ic_mean=0.9600, annual_return=0.8408 (anchor unchanged — no behavior drift).
• git diff --check main...HEAD → clean; merge-marker scan → 0.
• Secret scan over changed files: real .config.json token value 0 hits;
  FAKE_TUSHARE_TOKEN… only in the test fixture; .config.json/tushare.token
  only as config-key/scan-target descriptions.

Explicit non-goals (deferred to D2+)

• no TushareCache internal split;
• no endpoint schema registry;
• no data-quality validator;
• no concurrency / thread pool / async fetching;
• no CoverageLedger storage change;
• no PanelStore append/partition feature;
• no factor/alpha/portfolio/runtime/report metric changes.