Repin TestScaleEquivariance[placebo] baseline to Linux/OpenBLAS value

Swap the captured SE from 0.293840360160448 (macOS Apple Accelerate
local) to 0.2938403592163006 (Linux OpenBLAS, ubuntu-24.04-arm CI
runner). The warm-start landed in the prior commit threads
``unit_weights`` (a fit-time FW output that carries sub-ULP BLAS
reduction-order divergence) into per-draw FW init; across 200 draws
with path-dependent sparsification the SE diverges ~1e-9 between the
two platforms — exceeding the existing ``1e-12`` bit-identity gate.

CI is the gating surface. macOS local fits will now drift at ~1e-9
on this fixture; the inline comment documents that the delta is
finite-iter FW path-dependence, not a numerical regression.

ATT and empirical p-value are unchanged: ATT comes from the
deterministic FW solve (platform-stable to <1e-14 on this panel) and
the placebo p-value is integer-driven (1/(B+1) = 1/201 with no
placebo τ exceeding |ATT|).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: igerber

tests/test_methodology_sdid.py

@@ -3000,8 +3000,16 @@ class TestScaleEquivariance:
         # pass produces different sparsification under uniform vs warm
         # init on a handful of draws. Warm-start matches R at machine
         # precision (test_placebo_se_matches_r in
-        # TestJackknifeSERParity).
-        "placebo":   (4.603349837478791,   0.293840360160448, 0.004975124378109453, 200),
+        # TestJackknifeSERParity). Capture: Linux/OpenBLAS (CI runner) —
+        # warm-start carries ``unit_weights`` (fit-time FW output) into
+        # per-draw init, which is platform-divergent at sub-ULP from
+        # BLAS reduction order; across 200 draws with path-dependent
+        # sparsification the SE diverges ~1e-9 between Apple Accelerate
+        # (macOS local: 0.293840360160448) and OpenBLAS (Linux CI:
+        # 0.2938403592163006). Linux value pinned because CI is the
+        # gating surface; macOS local fits will drift at ~1e-9 — that
+        # delta is finite-iter FW path-dependence, not a numerical bug.
+        "placebo":   (4.603349837478791,   0.2938403592163006, 0.004975124378109453, 200),
         # bootstrap = paper-faithful refit with R-default warm-start: FW is
         # initialized with ``sum_normalize(unit_weights[boot_control_idx])``
         # for ω and with the fit-time ``time_weights`` for λ on each draw,