[KR] Add South Korea holiday calendars (national + market + settlement)

[davejoyce]

Module(s) Affected

holiday-calendar-apac

Problem / Motivation

GA Release 2.2.0 adds South Korea holiday calendars to holiday-calendar-apac, following the project's current national-vs-market-vs-settlement naming convention (ISO 3166-1 alpha-2 for national, ISO 10383 MIC for exchanges, ISO 4217 for settlement), consistent with the #229/#231-#241 remediation being applied elsewhere in the codebase. Building KR correctly split from day one avoids introducing the same conflation bug that work is paying down for other countries.

Per repo convention (git history for CN/CNY, SG/SGD, UK/GBP, and the recent UK/XLON split all shipped as separate, independently-reviewable issues, not one bundle), this parent tracks three sub-issues, one per calendar code.

Proposed Solution

Parent tracking issue only — no code changes here. See sub-issues:

• #TBD_KR — KR national public holiday calendar (Regulations on Holidays of Government Agencies), including the daecheday (substitute holiday) system
• #TBD_XKRX — XKRX Korea Exchange (KRX) market calendar
• #TBD_KRW — KRW Bank of Korea (BOK-Wire) settlement calendar — blocked on primary-source data, tracked so the gap is visible rather than silently dropped

KR and XKRX share the vast majority of their holiday content via a package-private KRHolidays factory class (mirroring the existing IsraelHolidays pattern that serves both IL/ILS in holiday-calendar-mena), so implement KR first — XKRX depends on it.

Alternatives Considered

Considered shipping KR as a single bundled issue (national + market + settlement together). Rejected for the same reason as the parallel Hong Kong decision: git history shows the project's actual convention is separate issues per calendar code, and bundling here would cut against the discipline #229 is currently establishing.

Additional Context

• Research package, engineering design, test plan, and devil's-advocate review for this work are recorded in the project's session notes; see sub-issues for the concrete per-code detail.
• Related: Separate national holiday calendars from equities market calendars (CRITICAL for GA) #229 (root national-vs-market remediation), [APAC] Split SGX from SG national calendar; redocument JP as national-only #232 (APAC module's own conflation fix, extracting XSES from SG).