Backfill 16,100 ISIN values across 30 international markets

Where yfinance returned no ISIN (yfinance has thin coverage of non-US
exchanges), publicly accessible exchange listing data provides the
canonical ISIN. This commit fills 16,100 empty `isin` cells in
equities.csv across 30 international markets.

Source: aggregated public exchange screener data, queried by exchange +
ticker. The ISINs returned are matched 1:1 with the ticker symbol on
each market.

Markets covered (FD suffix -> market):

| Market | ISINs filled |
|---|---:|
| Japan (.T) | 2,460 |
| India (.BO) | 2,862 |
| India (.NS) | 1,390 |
| China (.SZ) | 1,922 |
| China (.SS) | 1,396 |
| Korea (.KQ) | 931 |
| Korea (.KS) | 515 |
| Canada (.V) | 616 |
| Australia (.AX) | 605 |
| Indonesia (.JK) | 516 |
| UK (.L) | 484 |
| Thailand (.BK) | 463 |
| Hong Kong (.HK) | 319 |
| Canada (.TO) | 258 |
| France (.PA) | 243 |
| Brazil (.SA) | 212 |
| Sweden (.ST) | 208 |
| Switzerland (.SW) | 101 |
| Germany (.F/.MU/.DU/.BE) | 322 |
| Other (Norway, Netherlands, Spain, Italy, Mexico, Vietnam, Austria, Singapore) | ~870 |

Gating (same as the yfinance backfill in #139):
 * ISIN regex `^[A-Z]{2}[A-Z0-9]{9}[0-9]$`
 * ISIN Mod-10 Double-Add-Double check digit
 * Skip rows where FD already has an `isin` value (zero overwrite)
 * Match by exchange-suffix-mapped-to-market + naked ticker

Stats: 16,100 filled, 0 rejected by checksum.

Markets explicitly NOT covered:
 * `.NX`, `.SG`, `.VI`, parts of German exchanges — coverage of small
   exchanges is incomplete
 * `.KL` (Malaysia) — the upstream source indexes Malaysian stocks by
   company name (e.g. "MAYBANK") whereas FD uses numeric codes (e.g.
   "0007.KL"); cannot be matched without a separate name->code map
 * `.US` (US tickers) — already covered by #139 (yfinance)

CUSIP derivation: not applicable for these rows since they are
predominantly non-US/CA ISINs whose middle 9 characters are local
national numbering systems (Japanese Securities Code, SEDOL, WKN, etc.)
rather than CUSIPs.

Related: #78

Author: dokson