T20 — Schur complement mode #20

Task T20 of TASKS.md — milestone M8 — Schur complement mode.

Depends on: #19 (T19) must be merged first; tasks are strictly sequential.

Summary (from TASKS.md): Constrained ordering, unfactorized root front, schur_shape, dense and CSR schur_matrix, solve_fwd_schur/solve_bwd_schur; the three 5×5 examples of ../CUDSS.jl/docs/src/schur_complement.md pass on CPU and CUDA (test_schur_cudss.jl ported).

Scope and deliverables: the section T20 of TASKS.md on main is the single source of truth; it is not copied here so it cannot drift. Read it together with the PLAN.md sections it names and the Report blocks of the previous tasks.

Definition of done:

• Every test listed in the task passes on the KA CPU backend (GitHub runner) and on the CUDA/AMDGPU self-hosted runners (Run tests and Aqua checks green on the PR).
• The task's status marker in TASKS.md is set and its Report block is filled in honestly (CUDA counts come from CI).
• A PR titled T20: … from branch task/T20-<slug> with Closes #20 in its body, reviewed by the Claude reviewer with VERDICT: APPROVE.
• Out-of-scope bugs or follow-ups found on the way are opened as separate issues (label found-by-agent), not fixed silently.

How it starts: add the label claude:implement (the pipeline does this automatically after the previous task's PR merges; label on-hold to prevent that).

Generated by Claude Code