Add configurable spread neighborhood radius (SpreadRad)

[DLWoodruff]

Summary

Adds a --SpreadRad option that controls how far a burning cell can send spread messages in one neighborhood-layer definition. This ports the SpreadRad feature already merged in cell2fire/Cell2Fire#136 and proposed in fire2a/C2FK#3, adapted to C2F-W's code.

• --SpreadRad 1 (default): immediate neighborhood — identical to current behavior
• --SpreadRad 2: one extra ring; in general (2n+1)² − 1 candidate neighbors for radius n

The model keeps the original assumption that outgoing spread from a source cell i uses the ROS computed at source cell i for all candidate destinations.

Implementation notes

adjacentCells() takes a radius and returns the in-bounds neighbor box. Because a radius > 1 makes several neighbors share the same integer bearing, the old angle-keyed bookkeeping had to change:

• ROSAngleDir, angleDict, fireProgress, distToCenter are now keyed by neighbor id (previously some were keyed by integer angle).
• the angleToNb map is removed; a new selectHeadCell(windAzimuth) picks the neighbor closest to the wind azimuth for the slope calculation (replacing angleToNb[waz]).
• the cleaning step in Cell2Fire.cpp removes a burnt cell from each neighbor's maps by cell id.

initializeFireFields() and adjacentCells() gain the radius parameter; --SpreadRad is parsed in ReadArgs (default 1, clamped to ≥ 1) and echoed in the run output.

Verification

Built with g++ (c++14) and run on test/model/kitral-asc (--sim K, seed 123):

• --SpreadRad 1 reproduces the pre-change binary's burned-cell counts exactly (e.g. 2,18,21,8,9,…), so the default path is unchanged.
• --SpreadRad 2 spreads further (total burned ~403 → ~748 over 30 sims on the kitral instance).
• Adds test/spreadrad_test.sh (asserts default == --SpreadRad 1, that --SpreadRad 2 is reported, and that radius 2 burns ≥ radius 1) and wires it into build-debian.yml. The script passes locally.

I have not looked at the effect on any high-wind fires, which someone should do before you merge this.

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