P2-16 NativeEngine sparse MNA solver + factor reuse

[Kinin-Code-Offical]

(Synced from docs/current_status/issue_ready_backlog.md)\n\n# P2-16 NativeEngine sparse MNA solver + factor reuse\n\nSummary

• Area: NativeEngine circuit
• Deliverable: sparse matrix representation, LU reuse, benchmark harness.

Acceptance Criteria

• Sparse matrix implementation replaces dense solve for relevant paths.
• Factor reuse is implemented where safe.
• Benchmarks show improvement on representative circuits.

Test Plan

• Benchmark: compare runtime vs dense solver on multiple sizes.
• Correctness: compare node voltages within tolerance.

Likely File Targets

• NativeEngine circuit solver implementation

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