v0.3.0 — Shared rivernetworkx core and module modernization

This release migrates the toolkit onto a single shared river-network library,
modernizes the modules, and makes several correctness fixes. The API is still
pre-1.0 and may change.

Added

• rivernetworkx: a new importable Python package (pure NetworkX, no GRASS)
  that single-sources the river-network logic — graph construction
  (build_network/build_graph), raster sampling, breadth-first traversal,
  cumulative-distance accumulation, per-segment smoothing and densification,
  channel slope (−d z/d s) and slope–area, and node-link
  JSON I/O. The GRASS modules are now thin consumers of it, and the same
  representation is reusable outside GRASS (e.g. the GRLP coupling).
• v.stream.network gains an optional json= export (with elevation=,
  accumulation=, accum_mult=): it builds the NetworkX river-network graph,
  samples rasters along each segment, computes cumulative distance upstream of
  the outlet, and writes node-link JSON. This is the capability formerly
  provided by the separate v.stream.networkx module.
• v.stream.network and v.stream.profiler gain a -c flag (the region
  contains the full basin): a negative-accumulation channel head is then treated
  as an r.watershed boundary artifact (a divide on the region edge) rather than
  off-map inflow, skipping the off-map error.

Changed

• v.stream.profiler rebuilt on rivernetworkx. It walks either downstream
  to the outlet or upstream through all tributaries, samples
  elevation/accumulation/slope along the channel, optionally densifies
  (dx_target=) and smooths (window=) per segment (smoothing and
  densification never cross tributary junctions, preserving the natural breaks),
  and outputs a text long-profile table and/or a node-link JSON sub-network
  (json=).
• Drainage area sampled from r.watershed is returned as a positive magnitude
  (the outlet cells, which GRASS marks negative as a boundary flag, are
  recovered).
• examples/clean_coarsen_network.py now post-processes the
  v.stream.network json= export; its accumulation array is optional.

Fixed

• v.stream.profiler was non-functional on current GRASS GIS and pandas (a
  stale grass.raster import and a Series→int coercion); it runs again,
  and its long-standing branching bug is fixed by construction.
• v.stream.network writes the off-map outlet tostream as 0 (an earlier
  partial migration wrote -1); diverging (distributary/braided) networks now
  stop with a clear error instead of writing a partial tostream.
• Raster sampling is limited to the network's extent (covers the whole network
  even when the current region does not, and avoids reading a full large
  region), includes points exactly on the east/south region edges, and tolerates
  coincident vertices.
• Incomplete catchments (off-map upstream contributing area, flagged by negative
  r.watershed accumulation reaching a channel head) are caught with a clear
  error rather than silently producing a misleading network; the -c flag
  overrides this when the region truly holds the full basin.
• Raster NULL cells are read as NaN rather than a silent 0, so missing data
  stays honest; sampling accumulation with no data under a channel cell now
  errors clearly instead of slipping through as a fake 0.
• export_json emits standards-compliant JSON (non-finite floats as null, not
  the bare NaN token that strict parsers reject); load_json restores them.

Removed

• v.stream.networkx: retired. Its sole job (build graph → JSON) is now
  the json= option on v.stream.network, backed by rivernetworkx.
• v.stream.profiler/RiverNetwork.py: the bespoke network class is replaced
  by the shared rivernetworkx library.

Full changelog: v0.2.0...v0.3.0