feat(proxy): arbitrary-coordinate tide via KHOA model + noise guard

Adds a /api/tide?lat&lon path that derives high/low from the KHOA ocean-grid
model zeta for any coordinate, as a validated extension over the 61 official
stations:
- domain/tideExtrema.ts: parabolic extremum extraction + noise guard
  (extrema count > 6 or daily range < 0.3m => noisy). Unit-tested with the
  real 인천 (clean) and 묵호 (noisy) zeta series from the 2026-06-05 gate.
- adapters/khoaModel.ts: session cookie, anchor detection via pre_date=0,
  date->day-index, POST, zeta -> TideDay(source=model, datum=MSL, relative).
- domain/nearest.ts: nearest official station (Haversine) for fallback.
- routes/tide.ts: coord path with 1km grid rounding + 6h cache; falls back
  to the nearest official station when the model is noisy or KHOA fails.
- types.ts: TideDay gains source/datum/relativeLevel/noisy/fallbackFrom.

Verified live over HTTP: 인천 coord -> model (4 clean events), 묵호 coord ->
fallback to 묵호 official, 군산 station -> official (regression intact).

Constraint: KHOA ocean-grid is unofficial — model is an extension layer only; official 61 stations remain the absolute-cm source of truth and the fallback target.
Confidence: high
Scope-risk: module
Reversibility: moderate
Directive: Tune guard thresholds if South-coast moderate-tide points over-fallback; never make the model a sole dependency.
Tested: vitest 24/24 (incl. tideExtrema, khoaModel); live HTTP on all three paths against the deployed proxy.
Not-tested: bay-interior non-official points (no oracle); multi-day window reproduction; behaviour when KHOA rotates the model anchor mid-day.
Related: docs/BACKLOG.md 3-A, docs/checklist.md

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