Clean up exchange/market column inconsistencies + add consistency test

Per maintainer feedback on #143: keep `exchange` and `market` columns
in lock-step. Fixed 87 rows where the short code and the human-readable
label did not agree.

Category A — exchange code non-canonical, market is correct (84 rows):
  * NAS (12 rows, market "NASDAQ Global Select")    -> NMS
  * NYS (72 rows, market "New York Stock Exchange") -> NYQ

Category B — market label is wrong, exchange is correct (3 rows):
  * BTS-CATG (Leverage Shares 2X Long CAT Daily ETF on BATS BZX):
      market "OTC Bulletin Board" -> "BATS BZX Exchange"
      (Verified via Leverage Shares / Robinhood / Benzinga.)
  * NCM-CAPS (Capstone Holding Corp. on NASDAQ Capital Market):
      market "OTC Bulletin Board" -> "NASDAQ Capital Market"
  * NSI-LT.NS (Larsen & Toubro on NSE India, .NS suffix confirms):
      market "Metropolitan Stock Exchange" -> "National Stock Exchange of India"

After the fix every exchange code in equities.csv maps to exactly one
market label.

Added `tests/test_equities.py::test_exchange_market_one_to_one` that
asserts the forward direction (exchange -> 1 market) so future PRs
cannot silently re-introduce the kind of drift fixed here. The reverse
direction is intentionally not asserted: a market label may legitimately
cover several exchange tiers (e.g. "OTC Bulletin Board" covers
PNK / OQB / OID / OEM / OQX).

Author: dokson