[HK] Add Hong Kong holiday calendars (national + market + settlement)

[davejoyce]

Module(s) Affected

holiday-calendar-apac

Problem / Motivation

GA Release 2.2.0 adds Hong Kong 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) that the #229/#231-#241 remediation is standardizing across the rest of the codebase. Building HK correctly split from day one avoids repeating the conflation bug that work is currently paying down for US/CA/UK/AU/FR/SG.

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:

• [HK] Add Hong Kong national holiday calendar #257 — HK national statutory holiday calendar (Cap. 149 General Holidays Ordinance)
• [XHKG] Add Hong Kong Stock Exchange (HKEX) market holiday calendar #258 — XHKG Hong Kong Stock Exchange (HKEX) market calendar
• [HKD] Add HKMA/HKICL settlement holiday calendar (blocked — needs primary-source data) #259 — HKD HKMA/HKICL (CHATS) settlement calendar — blocked on primary-source data, tracked so the gap is visible rather than silently dropped

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

Alternatives Considered

Considered shipping HK as a single bundled issue (national + market + settlement together). Rejected: git history shows the project's actual convention is separate issues per calendar code (CN/CNY, SG/SGD, UK/GBP all shipped independently, sometimes released far apart), and the project is simultaneously running #229 specifically to un-bundle national/market conflation elsewhere — bundling a brand-new pair here would cut against that same discipline.

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).