docs: add 3-A KHOA model (arbitrary-coordinate tide via ocean-grid)

Documents the discovered KHOA ocean-grid internal API
(selectListModelTableDayData.json) that returns hourly zeta/current/
temperature/salinity for arbitrary lon/lat — the likely path to the
user's core need ("nearby ports"), beyond the 61/87 public-data ceiling.
Records the advisor's height/timing split strategy (official 61 stations
keep absolute cm; model trusted only for timing/range/multtae/current/
temp because the MSL→DL datum constant Zo varies per point) and the
remaining mandatory verification gate (East-coast small-tide stations,
bay interior, multi-station × week × 4 events).

Constraint: KHOA ocean-grid is an unofficial internal API — never a sole dependency; proxy must cache + degrade to the 61 official stations.
Confidence: medium
Scope-risk: local
Reversibility: trivial
Directive: Run the real verification gate (묵호/포항 + bay interior) before wiring arbitrary-coordinate tide into the live app.
Tested: Not applicable (doc).
Not-tested: KHOA model timing accuracy on small-tide East-coast stations (gate pending).
Related: docs/BACKLOG.md

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