Fill 17,252 empty exchange + market cells in equities.csv

Two passes:

**Pass 1 (deterministic, 17,240 rows)**: for every row with `exchange`
empty AND a recognisable ticker suffix (`.NX`, `.F`, `.DU`, `.MI`, ...),
derive the canonical exchange code from rows that already have that
same suffix populated. No external API. The mapping is unambiguous at
>=99% per suffix (verified before applying):

| Suffix | Exchange | Rows |
|---|---|---:|
| .NX | ENX (Euronext) | 11,364 |
| .F | FRA (Frankfurt) | 3,068 |
| .DU | DUS (Dusseldorf) | 624 |
| .BE | BER (Berlin) | 370 |
| .MI | MIL (Borsa Italiana) | 349 |
| .MU | MUN (Munich) | 247 |
| .SG | STU (Stuttgart) | 218 |
| .OL | OSL (Oslo) | 115 |
| .TI | TLO (EuroTLX) | 115 |
| .ST | STO (NASDAQ OMX Stockholm) | 89 |
| .BO | BSE (BSE India) | 75 |
| .PA | PAR (Euronext Paris) | 70 |
| .VI | VIE (Vienna) | 68 |
| .MX | MEX (Mexico) | 63 |
| Other suffixes | various | ~405 |

**Pass 2 (external resolution, 12 rows)**: for the 98 remaining rows
(US-style symbols with no suffix and every other column NaN — failed
scrapes), query yfinance and Finnhub. yfinance resolved 9; Finnhub's
US universe endpoint resolved 3 more (MIC mapping: XNYS->NYQ, XNAS->NMS,
OOTC->PNK). TradingView returned no additional matches on this set.

Stats:
 * 17,240 deterministic suffix-based fills
 *     12 external resolution fills (9 yfinance + 3 Finnhub)
 * 17,252 `exchange` cells filled (was: 17,338 empty before)
 * 17,252 `market` cells also filled (inherited from canonical
                                      `exchange -> market` mapping)
 *     86 rows still empty: US-style symbols not found in any free
        data source (delisted warrants/units like `BFT-WT`, `CAS'U`,
        `CCX'U`, etc.). Out of scope.

Validation: passes `test_exchange_market_one_to_one` (introduced in
JerBouma#143) — every exchange code in `equities.csv` still maps to exactly
one market label.

No row with a pre-existing populated `exchange` was modified. Only
empty cells filled.

Author: dokson