Add landuse module (ESA WorldCover) with Manning roughness

[rehsani]

Summary

• New `floodpath.landuse` module: ESA WorldCover 10 m global land-cover via `/vsicurl/` window reads on the public AWS Open Data S3 bucket (eu-central-1). Supports 2020 v100 and 2021 v200 releases.
• `landuse_to_roughness` + `RoughnessGrid` derive Manning's n from any categorical landuse raster. Default `WORLDCOVER_MANNING_N` mapping uses Chow 1959 Table 5-6 / USACE EM 1110-2-1417 floodplain values (n=0.025 for water/bare, n=0.150 for dense tree cover); fully overridable for site-calibrated work.
• One small unrelated safety fix bundled: `.pypirc` added to `.gitignore`.

Module surface

Item	What
`get_worldcover_landuse(lat, lon, buffer_deg, year)`	Fetch a WorldCover patch as a `LanduseGrid`
`LanduseGrid`	Categorical raster + `class_counts()` / `class_name()` / `fraction()`
`landuse_to_roughness(landuse, mapping=None, fallback=0.035)`	Pure transformation to Manning's n
`RoughnessGrid`	n raster + active mapping + fallback (introspectable)
`WORLDCOVER_CLASSES`, `WORLDCOVER_MANNING_N`	Default reference tables

Test plan

• `pytest -m "not integration"` — 154 passed (137 prior + 17 new roughness tests, plus the existing 28 landuse tests; 13 deselected integration)
• `pytest -m "integration" tests/landuse` — 3 passed: live ESA fetch in Djibouti for both 2020 and 2021 + Robit Bata fixture regression check
• End-to-end smoke test on Robit Bata: 13 stages all succeed, dominant class `cropland (86.2%)`, mean n=0.0455 matches pinned test values
• Committed fixture is 40 KB (`tests/fixtures/robit_bata_worldcover.tif`, 900x900) — well under the per-fixture budget

Notes

• `get_worldcover_landuse` uses `/vsicurl/` (DEM-style) rather than the GHSL-style download-and-cache, because WorldCover tiles are 3deg x 3deg at 10 m (~300 MB each) and the S3 bucket honours HTTP range requests. Caching whole tiles for one bbox would be wasteful.
• README modules table updated; full quickstart snippet for landuse intentionally deferred to a follow-up commit alongside other docs work.