Core correctness: CovMatrix stride, eigenvector guard, std::abs

[gavinsdavies]

Part of #303.

Fixes

RecoBase/DwnstrTrack.h — covariance stride mismatch. CovMatrix(i,j) always read fCovXY[5*i+j], but for no-magnet 4-parameter tracks Reset(true) resizes fCovXY to 16 and the producer (SSDRecDwnstrTracksAutre) fills it with stride 4 (SetCovarianceMatrix(i*nPars + j)). So every off-diagonal read on 4-param tracks returned the wrong element, and (3,3) read index 18 of a 16-element vector (out of bounds). The getter now uses NumParams()*i + j, matching the setter. A full call-site audit (all two-argument CovMatrix( uses are in SSDDVertexFitFCNAutre.cxx) confirmed all callers pass logical indices and none compensates for the old stride; the 5-parameter/magnet path is byte-for-byte unchanged. No other RecoBase class has this pattern (VertexAutre uses a true 2-D array).

RecoUtils/RecoUtils.cxx — degenerate eigenvector guard. findLine divides by the principal eigenvector's z-component without a guard; an xy-collinear point set gives n[2] ≈ 0 and infinite endpoints. Added an early return (epsilon 1e-12 on a unit-normalized vector — far below any physical slope, above eigen-decomposition round-off) in the same style and output contract as the existing el == -1 check: lfirst/llast stay zero-initialized, which the callers in SingleTrackAlgo already tolerate.

Geometry/Geometry.cxx — portable std::abs. Detector::View() used unqualified abs(sin(...)) on doubles (×4). It currently resolves to a floating overload under GCC/libstdc++, but if it ever resolves to C int abs(int) (other toolchain/include order), every comparison < 0.2 collapses and all sensors classify as X_VIEW. Now std::abs, with an explicit <cmath> include.

Verification

The CovMatrix change is a deliberate correctness change for 4-param tracks only (audit in the description above); everything else is behavior-preserving. No local build environment and no repo CI — a collaborator build before undrafting would be appreciated.

🤖 Assisted by Claude Code (claude-fable-5)

[FNALbuild]

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