Chart prior importer: contour depth-below-surface → ellipsoidal Chart tiles (A2 of unh_marine_autonomy#163)

[rolker]

A2 of rolker/unh_marine_autonomy#163 (the second of two PRs). A1 — the core
SourceLayer::Chart (read-only prior) — is merged
(rolker/unh_marine_autonomy#165). This issue is the importer that converts
the existing contour prior into ellipsoidal Chart tiles the store can load, and
closes rolker/unh_marine_autonomy#163 when merged.

Why here

The store core must stay datum-agnostic (it's in core_ws). The conversion
belongs in the importer, which lives above platforms_ws — cube_bathymetry
(sensors_ws) can link both marine_bathymetry_store (core) and
mru_transform's datum_config (platforms). Keeps ADR-0002 §D7 (convert at
import) and the layer rules both satisfied.

What to build

1. Chart-import path. Read the raw contour tiles
   (~/data/bathymetry/massabesic/store/chart/<date>/, 142 level-11 GeoTIFFs;
   band 1 = depth below lake surface, negative-down, metres; NH GRANIT 5-ft
   contours; band 2 uncertainty const 3.0; band 3 timestamp). For each cell,
   convert to WGS84 ellipsoidal height:

   ellipsoidal = chart_datum_z + depth_below_surface
   

   sourcing chart_datum_z per tile from
   mru_transform::resolve_datum(lat, lon, entries, /*vdatum=*/nullopt). Write
   the result into a marine_bathymetry_store store opened chart_writable=true
   (the A1 API), then save() it to the store's chart/ layer.

2. Datum entry. Add a massabesic polygon DatumEntry
   (chart_datum_z: 52.3) to the datum config chart_datum_node loads — the
   single source of truth for the lake-surface ellipsoidal height. (52.3 m =
   estimated WGS84 ellipsoidal water-surface height; same value the sim uses.)

Verify

• A −13.716 m (45 ft) contour cell imports to 38.58 m ellipsoidal
  (52.3 − 13.716); runtime clearance z(map_tide) − seafloor = 52.3 − 38.58 = 13.72 m = water depth.
• All 142 tiles covered; re-import is idempotent (content-hash unchanged).
• The produced store loads into a default (read-only-Chart) store and a
  best-source query returns the prior where no Draft/Processed exists.

Depends on / notes

• A1 (merged): SourceLayer::Chart + chart_writable construction flag.
• mru_transform/datum_config.hpp (resolve_datum, DatumEntry).
• Lake = constant offset (off-VDatum-coverage fallback); ocean VDatum path is a
  separate deferred issue.
• Feeds the sim data harness (Sim bathy data harness: reproducible build of boat-prior + MBES-truth stores unh_marine_simulation#76) and the Phase-4
  costmap layer (Phase 4: bathymetry_layer Nav2 costmap plugin (store-backed clearance, lazy tiles) unh_marine_autonomy#164).

Part of rolker/unh_marine_autonomy#163

---

Authored-By: Claude Code Agent
Model: Claude Opus 4.8 (1M context)