docs: plan v4.3 and correct the stale roadmap status notes

[backlundtransform]

The autumn roadmap had drifted from the code. LinearAlgebra Phase 1-2 was merged but still described as an open PR, a customer-driven physics track had taken over the autumn without appearing in the plan at all, and v4.1.0 and v4.2.0 were tagged with contents that do not match their release tables.

Add Roadmap-v4.3.md, which plans the next release around the debt Phase 1-2 left behind: the decompositions were built but their call sites were never migrated, so eight files still solve dense systems with hand-rolled Gaussian elimination and CoupledOscillators keeps a private Jacobi eigensolver. Two of those migrations are accuracy fixes rather than cleanup - FittingSolver squares its condition number by going through the normal equations, and the Kalman filters form an explicit inverse of a symmetric positive-definite innovation covariance. The release also adds the root-finding module, whose absence leaves NewtonRaphson as the only root finder: it runs a fixed 100 iterations with no convergence check, no guard against a vanishing derivative, and a fresh Pascal matrix allocated per iteration.

Mark the autumn plan as background rather than an active plan, and correct LinearAlgebraRoadMap where it no longer matched the code: Phase 1-2 is merged, and Phase 4 is partly done outside the plan - SparseMatrix has CSR storage and a Jacobi-preconditioned CG that drives Assembler2D, verified against the existing sparse and 2-D FEM tests.

Documentation only; no code changes.