Return scalar ngspice operating points

[ShiboSoftwareDev]

Summary

• convert real ngspice .op vectors with no time axis into scalar voltage/current Circuit JSON records
• preserve probe names and source metadata, including differential voltage probes and current-sense aliases
• add caller-configurable timeouts and reset the cached engine after a timeout
• classify convergence, timeout, model, unsupported-analysis, netlist, and engine failures
• reject absent or out-of-scope analysis directives explicitly while keeping transient graph behavior unchanged

Stack and merge order

1. Schema/result contract: add operating-point simulation result schema circuit-json#646
2. Public props: Add operating-point simulation props props#730
3. .op netlist generation: Generate DC operating-point SPICE analyses circuit-json-to-spice#50
4. ngspice execution/result conversion (this PR): Return scalar ngspice operating points #25
5. SVG results and diagnostics: feat: render DC operating-point simulation results circuit-to-svg#622
6. Core integration: feat: add DC operating-point simulations core#2682

Validation

• bun test tests/*.test.ts (29 passing, including real ngspice OP/transient runs and existing SVG snapshots)
• focused PSPICE compatibility fixtures pass with the repository timeout raised for their CDN startup
• bun run typecheck
• bun run build
• bun run format:check
• Biome check on every changed file

The repository-wide bun run lint also reports two pre-existing import-order findings in untouched files (lib/eecircuit-result-to-simulation-graphs.ts and tests/eecircuit-result-to-vgraphs.test.ts).