engine: compile_project_incremental deep-clones CompiledSimulation out of the Arc on every call #643

Problem

After the assembly-memoization change, assemble_simulation returns Arc<CompiledSimulation> (a salsa-tracked memo). But compile_project_incremental (src/simlin-engine/src/db.rs) does (*arc).clone() to return an owned CompiledSimulation, preserving its public signature.

The big win lands: a no-op recompile correctly skips assembly (the memo is reused). But the call still deep-clones the entire CompiledSimulation -- the modules / offsets / cached_constant_info maps -- on every call, even when nothing changed.

Why it matters

The deep clone is pure overhead on the hot recompile path. Returning Arc<CompiledSimulation> to callers would drop even that clone. The downstream callers (libsimlin / pysimlin / cli) construct Vm::new by value, so threading the Arc through (or having Vm::new accept &CompiledSimulation / Arc<CompiledSimulation>) would eliminate the clone end-to-end.

Not a correctness issue

This is a performance follow-up only -- results are unaffected.

Components affected

• src/simlin-engine/src/db.rs (compile_project_incremental)
• Callers constructing Vm::new: libsimlin, pysimlin, simlin-cli

Severity

low / perf.

Context

Identified during the salsa-pipeline-cleanup refactor (the assembly-memoization change that introduced Arc<CompiledSimulation> from assemble_simulation).