test: add oblique (2,1) shear-wave diagnostic with composite modal fit

Test-only. New shear_wave.py (exact field, equilibrium initializer reusing
frozen oracle constants/geometry, area-weighted composite metrics with a
2x2 Gram-matrix modal fit, late-time rate fit, preregistered assess with
max(2*control, floor) budgets). run_case.py gains --flow (default
taylor_green, unchanged receipt). oracle.py byte-identical (pinned by test).
CPU manufactured checks in test_shear_wave.py; docs record caveats.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/validation/taylor_green.md

@@ -159,3 +159,87 @@ theoretical stress-consistency certification or production-kernel change.
 ## Repository integration
 
 The four Python harness files remain together under tests/validation/taylor_green, so runtime provenance hashes precisely this harness directory. Formatting changes produce a new harness identity; preparation and integrated receipts must not be mixed. All analytic definitions, numerical floors, assessment gates, sample times, and source-import assertions are unchanged. Production source is unchanged from the cd3 snapshot; these periodic tests do not waive the separate channel GPU gate.
+
+## Oblique shear wave (test-only diagnostic, not GPU-qualified)
+
+`shear_wave.py` reuses the frozen oracle's constants, cell centers and
+geometry. `oracle.py` is byte-identical, and `test_shear_wave.py` pins its
+eec4677 SHA-256. `run_case.py --flow shear_wave` selects it. The default flow
+is still the frozen Taylor–Green oracle, and the default receipt is unchanged.
+
+Convention: integer mode (2,1) on the L=N periodic root domain, so
+`k = (2*pi/L)*(2,1)` points 26.565° from +x. Mode (1,2) would point 63.435°.
+A wrong-mode test catches a swap between the two. Polarization is
+`E = (-1,2)/sqrt(5)`, perpendicular to k:
+
+    u = U*E*sin(k.x + phi)*exp(-nu*|k|^2*t),  phi = 0,  p = const,  rho0 = 1
+
+Here `nu = U*N/100`, the Taylor–Green Re100 viscosity at the same N. The
+initial condition is FP64 second-order equilibrium at the actual leaf
+centers, with no non-equilibrium part.
+
+Physical caveats:
+1. *Finite-Mach error.* The second-order D2Q9 equilibrium has third moment
+   `sum_i f_eq c_a c_b c_g = rho*cs^2*(u_a d_bg + u_b d_ag + u_g d_ab)`. The
+   `(c.u)^2` term contributes only even moments. The continuum target also
+   has `rho*u_a*u_b*u_g`. At Navier–Stokes (first Chapman–Enskog) order,
+   the resulting stress error is proportional to `d_g(rho u_a u_b u_g)`.
+   For this field with uniform density, `u.grad = 0` and `div u = 0`, so
+   that particular term vanishes.
+
+   This does *not* show that all finite-Mach or cubic error vanishes. It
+   does not cover:
+   - higher Chapman–Enskog orders;
+   - lattice truncation for the oblique wavevector;
+   - FP32 storage;
+   - any density nonuniformity, which interfaces or anisotropy can create.
+2. *Startup.* Equilibrium initialization omits the non-equilibrium stress.
+   The resulting kinetic transient lasts O(tau) steps and may shift the
+   in-phase amplitude. The decay rate is fitted from `ln(a_hat)` using
+   only samples with t* >= 0.12, which is about 1.9N root steps and much
+   longer than tau. This is intended to reduce startup sensitivity. It
+   has not been shown to cancel the bias, and that remains unverified
+   until measured.
+
+Metrics are computed on ACTIVE leaf cells only, each weighted by its area
+`4**-level` in root-cell units. The partition must cover the domain exactly
+once. One global pressure gauge is used, never one per level.
+
+- `a_hat`, `b_hat`: weighted least-squares fit of `u.E` onto the sin and cos
+  modes, solved with the 2x2 Gram matrix of the actual composite quadrature.
+- `phase_drift = atan2(b_hat, a_hat)`, in radians.
+- `rate_error = |rate_fit/(nu|k|^2) - 1|`, dimensionless. Rates are per root
+  step.
+- `velocity_l2`: error relative to the exact field.
+- `residual_l2`: the velocity left after subtracting the fitted mode,
+  relative to the exact-field norm.
+- `longitudinal_l2`: the `u.k_hat` part of the velocity, relative to the
+  exact-field norm.
+- `pressure_rel = p_rms/(A(t)^2/4)`: the gauged composite rms of
+  `p = (rho-1)/3`, divided by a *fixed analytic scale*.
+  - A²/4 is the continuum rms of the Taylor–Green exact pressure at the
+    same amplitude. That makes the number comparable in magnitude with TG
+    `pressure_l2`, but it is not that norm. TG normalizes by the discrete,
+    globally gauged exact-pressure norm on the composite grid.
+  - Exact pressure here is constant, so no relative-to-exact claim is made.
+  - Max and per-level values are reported for localization.
+
+Preregistered screens (`shear_wave.assess`):
+- Run set: uniform N, uniform 2N, and refined depth-2 runs at root N, with
+  constant and/or projected_linear explosion.
+- `eps(N) = (|k| h_root)^2 = 5*(2*pi/N)^2`, dimensionless, coefficient one.
+  It is 0.01205 at N=128.
+- Absolute screens: rate_error, max |phase_drift| (radians) and max
+  residual_l2 must each be <= eps. This is a preregistered diagnostic
+  choice of the second-order truncation scale, *not* a proven error bound.
+- Existing ceilings also apply: velocity_l2 <= 0.05, mass <= 1e-5 and
+  pressure_rel <= 0.02.
+- Budget form, as in `assess.py`:
+  `refined <= max(2*control_N, floor)`, with floors 0.002 for rate and
+  residual and 0.02 for pressure.
+- The ratio to the 2N control is reported, not gated.
+- A control failing an absolute screen makes the set INVALID_CONTROL. The
+  screens are never retuned.
+
+Scope: one flow, one rectangle, SRT only. Two resolutions show a trend,
+not an order. This does not establish placement robustness.