Only apply the cyclostrophic deprecation shim when the argument is passed

[dylanpulver]

Changes proposed in this PR:

• Give the deprecated cyclostrophic argument of compute_angular_windspeeds a None sentinel default instead of False, so the deprecation shim only fires when a caller actually passes it.
• Copy model_kwargs before the shim writes into it, so a caller's dict is never modified in place.

This PR fixes #1209

compute_angular_windspeeds declares cyclostrophic: Optional[bool] = False but guards the shim with if cyclostrophic is not None:. Since False is not None, the shim runs on every call — including the internal one from compute_windfields_sparse, which never passes the argument. So a DeprecationWarning is raised for an argument nobody passed (the 44 occurrences in #1209), and model_kwargs["cyclostrophic"] = cyclostrophic then overwrites the caller's own setting with False. That makes cyclostrophic unreachable via model_kwargs, the exact route the deprecation message points users to.

Measured on develop with the ER11 setup from test_er_2011_pass (r_max = 40 km, v_max = 40 m/s, f = 6.61918149e-05 1/s), at r = 30 km:

call	v_ang (m/s)
_stat_er_2011(..., cyclostrophic=True)	38.400000
_stat_er_2011(..., cyclostrophic=False)	39.670883
compute_angular_windspeeds(..., model_kwargs={"cyclostrophic": True})	39.670883

The caller's dict comes back as {'cyclostrophic': False}. End to end, TropCyclone.from_tracks(tracks, centroids, model="ER11", model_kwargs={"cyclostrophic": True}) returns an intensity matrix bit-identical to the default call.

38.4 m/s is equation (36) of Emanuel and Rotunno (2011), M = M_max · 2(r/r_max)² / (1 + (r/r_max)²) with M_max = r_max·v_max = 1.6e6 m²/s: 1.6e6 · 2 · 0.5625 / 1.5625 / 30e3 = 38.4. The non-cyclostrophic branch adds 0.5·f·r_max² = 52953.45 m²/s to M_max, giving 39.670883.

No change to existing default behaviour: without the injected cyclostrophic=False, each model falls back to its own signature default — False for H1980, H08 and ER11, True for H10 — which is what TropCyclone.from_tracks already documents. H10 ignores the flag either way, so its spurious "this setting is ignored" log line also stops. No stored reference value changes; numbers move only for callers who explicitly asked for cyclostrophic and were previously ignored.

Introduced in cb97195. Existing tests call _stat_holland_1980 and _stat_er_2011 directly, so nothing covered the dispatcher's kwarg forwarding. Three tests added to TestWindfieldHelpers, asserting literals derived from Emanuel and Rotunno (2011) eq. (36) rather than from the code. test_trop_cyclone_windfields.py goes 9 passed → 12 passed, and all three fail against the unpatched file. Across test_trop_cyclone.py + test_trop_cyclone_windfields.py the count goes 18 passed → 21 passed with no regression, and the cyclostrophic deprecation warnings raised by that scope drop from 43 to 0.

Two checklist boxes are left unticked deliberately. test_cross_antimeridian was
deselected locally because it downloads the coast-distance grid from Zenodo, which
returned 504 here — CI caches that data, so it should run, but I have not proven it
green either way. pylint --rcfile=.pylintrc reports 9.52/10 on both changed files
with every message pre-existing and structural, and none introduced by this diff.

Written with AI assistance; the wind speeds above are function output, and 38.4 m/s
is hand-derived from the published equation.

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