Revisit `plan_read` mortie linestring vs shapely after mortie v1 (#57)

[espg]

Summary

Tracking follow-up from #53 (closed with Option A: keep shapely LineString in plan_read unconditionally for now).

When mortie v1 ships (#57), revisit whether plan_read's within-granule segment->AOI crossing check should branch on grid type and call into mortie's linestring coverage for HEALPix grids — for end-to-end S2 consistency with mortie-driven shard dispatch.

Why deferred (per #53 analysis)

• Euclidean error is negligible at ATL03 photon spacing. ~10 m per 100 km of segment, well inside any usable HEALPix cell — the within-granule check is a crossing filter, not a precision geometry op.
• Avoid premature binding to a moving API. mortie is mid-redesign for v1 (morton_index datatype per Major Feature / Enhancement — Full resolution Morton MOC datatype espg/mortie#35, etc.); the linestring coverage interface may shift. Binding plan_read now risks rewriting it once v1 lands.

When to revisit

After #57 (mortie v1 integration) resolves and the v1 linestring API surface is stable. At that point evaluate:

1. mortie v1 linestring coverage signature — what does the call look like; does it match plan_read's segment-rep-point input shape ((lat, lon) pairs)?
2. Grid-type branch in plan_read — is the right seam at plan_read(...) (passing a grid handle), or in _planned_read_group (computing the crossing mask before calling plan_read)?
3. Measurable benefit on a real region — does the S2 path actually change which segments match the AOI for a 10 km × 10 km HEALPix shard, vs the current Euclidean path? If parity is byte-identical at ATL03 cell scales, the consistency argument alone may not justify the dependency.

Refs

• Original discussion: read_plan: select mortie linestring vs shapely by grid type (HEALPix -> mortie) #53
• Phase C plan_read implementation: Generic hierarchical filtering & targeted reads (#43) #49 (and PR ATL03 template + region benchmark + hierarchical-read wiring (Refs #30, #43) #47's phase 5 wiring it into _read_group)
• Mortie v1 integration: Integration with pending version 1 of mortie #57
• plan_read source: src/zagg/read_plan.py