Linestring / multi-linestring morton coverage + metric buffer helper

[espg]

Summary

Adds mortie.linestring_coverage() — morton-index coverage of open polylines — and mortie.morton_buffer_meters(), a metric-width convenience wrapper around morton_buffer (from #17). Includes a real-data example notebook that queries the NSIDC InSAR Antarctic grounding line dataset. Closes #25.

What's new

linestring_coverage(lats, lons, order=18) — Python interface over a new Rust module (src_rust/src/linestring.rs). Rasterizes each segment between consecutive vertices by sampling the great-circle arc at half-cell-resolution spacing, guaranteeing a contiguous cell chain. Reuses the Phase A interpolation helpers from coverage.rs (made pub(crate)), so there's one source of truth for the vertex-neighbor logic introduced in #17.

Return shape:

• Single line (1-D arrays) → sorted, unique np.ndarray of morton indices.
• Multi-line (list of 1-D arrays) → list of arrays, one per input line. Lengths may differ; per-line results are not deduplicated across lines.

morton_buffer_meters(cells, width_m) — wraps morton_buffer. Infers order from the input cells, computes k = ceil(width_m / cell_width_m) using R_earth · sqrt(π/3) / 2^order, and returns the resulting border ring. The docstring is explicit that this is approximate: the achieved width is rounded up to the nearest whole cell width, so the buffer always covers at least width_m.

Why this API shape

Lat/lon array input (rather than shapely geometries) keeps mortie dependency-free. The example notebook shows the idiomatic pattern: earthaccess → geopandas → unpack shapely LineString / MultiLineString into arrays → linestring_coverage. The multi-line return is a list (not a 2-D array or single concatenated 1-D array) because per-line lengths differ, and callers who want the union can trivially do np.unique(np.concatenate(per_line)).

Files

• src_rust/src/linestring.rs — new Rust module with linestring_to_morton_coverage (10 unit tests).
• src_rust/src/coverage.rs — three Phase A helpers promoted to pub(crate). No behavior change.
• src_rust/src/lib.rs — rust_linestring_coverage pyfunction registered in _rustie.
• mortie/linestring.py — linestring_coverage Python wrapper, validation, multi dispatch.
• mortie/tools.py — morton_buffer_meters helper.
• mortie/__init__.py — public exports.
• mortie/tests/test_linestring.py — 27 tests (single/multi shape, sort/unique, endpoints present, interpolation fills gaps, hemisphere signs, order-range validation, NaN/inf rejection, meter-buffer monotonicity + equivalence to k).
• examples/morton_linestring_insar_gl.ipynb — self-contained example: synthetic linestring → real NSIDC-0498 v2 InSAR grounding line via earthaccess → south-polar-stereographic plot → buffer demo (k=1, k=3, ~50 km metric).
• pyproject.toml — new docs extra for the example notebook (jupyterlab, notebook, ipykernel, matplotlib, shapely, geopandas, pyogrio, earthaccess, cartopy, healpy, requests).

Test plan

• cargo test — 67 Rust tests pass (10 new linestring tests).
• pytest mortie/tests/ — 203 passed, 3 skipped (27 new linestring tests).
• Notebook smoke-checked: synthetic linestring renders; CMR search returns granules for NSIDC-0498 v2.
• End-to-end notebook run (Earthdata download + plots) — please verify while reviewing.

Clarifying Q&A

Design decisions and the implementation plan are documented as comments on #25 for reproducibility.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 98.43750% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.27%. Comparing base (b87f65e) to head (5da9f28).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
mortie/tools.py	87.50%	2 Missing ⚠️
mortie/linestring.py	96.15%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #26      +/-   ##
==========================================
+ Coverage   87.30%   88.27%   +0.97%     
==========================================
  Files          15       17       +2     
  Lines        2000     2192     +192     
==========================================
+ Hits         1746     1935     +189     
- Misses        254      257       +3     

Flag	Coverage Δ
unittests	88.27% <98.43%> (+0.97%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
mortie/__init__.py	80.00% <100.00%> (+2.22%)	⬆️
mortie/tests/test_linestring.py	100.00% <100.00%> (ø)
mortie/linestring.py	96.15% <96.15%> (ø)
mortie/tools.py	77.71% <87.50%> (+0.46%)	⬆️

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[codspeed-hq]

Merging this PR will degrade performance by 10.39%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 2 improved benchmarks
❌ 1 regressed benchmark
✅ 26 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	order6[100]	414.7 µs	363 µs	+14.23%
❌	test_coverage_triangle_order6	618.5 µs	690.2 µs	-10.39%
⚡	test_coverage_triangle_order4	307.2 µs	272 µs	+12.93%

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_{Comparing feature/morton-linestring-coverage (5da9f28) with main (b87f65e)}