Add Zhong 2008 propagator-matrix analytic response

[gthyagi]

Dependency

This is a stacked PR on top of #591, which adds the generic
uw.postprocessing.geoid coefficient API. The Zhong oracle calls those pure
functions rather than carrying a second geoid/self-gravity implementation.

Until #591 merges, this comparison also shows its three commits. After #591
merges into development, the remaining diff is the two commits
95b34701 and fb9ce95b.

What this adds

uw.analytic.Zhong2008 is a mesh-independent semi-analytical reference for the
spherical-shell delta-load Stokes benchmarks in Zhong et al. (2008).

For one harmonic degree l, it:

• propagates the four-component Hager--O'Connell poloidal state in
  v = log(r) with an exact matrix exponential in each constant-viscosity
  layer;
• accepts arbitrary piecewise-constant radial viscosity profiles;
• applies the radial delta load as the traction-state jump
  (0, 0, -rint * load, 0);
• solves the two remaining CMB state coefficients from impermeable free slip
  (radial velocity = tangential traction = 0) at both shell boundaries;
• returns no-self-gravity topography/geoid, signed characteristic horizontal
  velocity, and horizontal velocity divergence at the surface and CMB;
• returns self-gravity-corrected topography/geoid through the generic API from
  Add generic spherical-shell geoid and self-gravity postprocessing #591.

This is intentionally a numeric response oracle rather than an
AnalyticSolution(mesh): the paper publishes boundary response coefficients,
not pointwise symbolic velocity/pressure fields. It is exported from
uw.analytic but is not registered by available(), whose contract requires a
mesh-constructible symbolic solution.

Radial system

With L = l(l + 1) and state

u = (y1, y2, r sigma_rr / eta0, r sigma_r_perp / eta0)^T,

the constant-layer system is

du/dv = A u
[ -2 L 0 0 ]
[ -1 1 0 1/eta ]
A = [ 12 eta -6 L eta 1 L ]
[ -6 eta 2(2L-1) eta -1 -2 ]

and u(rb) = exp(A log(rb / ra)) u(ra).

API example

reference=uw.analytic.Zhong2008(
harmonic_degree=2,
radius_inner=0.55,
radius_outer=1.0,
internal_load_radius=0.775,
).response()
reference.surface_characteristic_velocityreference.self_gravity.surface_topography

The layered Table 3 case is selected with:

viscosity_interfaces=(0.971875,)
viscosities=(1.0, 1.0e4)

Published-reference validation

The focused test checks every parenthesized propagator entry in Zhong et al.
(2008) Tables 2 and 3:

Cases	Depths	Degrees	Quantities per case	Published values
Isoviscous	0.25d, 0.5d, 0.75d	2, 5, 8, 15	8	96
10^4 viscosity lid	0.25d, 0.5d, 0.75d	2, 5, 8, 15	8	96

All 192 values agree within the precision printed in the paper. Additional
tests cover both free-slip boundary states, response linearity in load
amplitude, no-self-gravity recovery, input validation, and public API behavior.

Representative degree-2, mid-mantle responses:

Case	s	b	h_t	h_b	U_t	U_b
Isoviscous oracle	0.499849	0.931260	0.0448629	0.0546149	-0.0100641	0.0118606
Paper	0.4998	0.9313	0.04486	0.05461	-0.01006	0.01186
Layered oracle	0.610374	0.435376	0.0578926	0.00675446	-1.25811e-05	0.00555605
Paper	0.6104	0.4354	0.05789	0.006754	-1.258e-05	0.005556

Checks

• python -m pytest tests/test_1029_analytic_zhong2008.py -v:
  34 passed
• analytic contract/optional/oracle group: 66 passed, 15 skipped
• ./uw test Level 1: 1476 passed, 49 skipped, 2 xfailed
• documentation audit still exits on the repository-wide baseline of 138
  undocumented API items; it reports no new missing Zhong API item.

Review focus

Please check:

1. the four-state matrix and traction scaling against the intended
   Hager--O'Connell convention;
2. the sign and rint factor in the delta-load jump;
3. whether a mesh-independent response oracle should remain directly exported
   but outside the symbolic available() registry;
4. the constructor representation of radial viscosity interfaces.

References

• Hager & O'Connell (1981), JGR 86, 4843--4878,
  doi:10.1029/JB086iB06p04843.
• Zhong et al. (2008), G3 9, Q10017, doi:10.1029/2008GC002048.