fix: drop unsupported subsystem_levels kwarg from build_problem calls

[aarontrowbridge]

Summary

Piccolo's SplinePulseProblem does not accept a subsystem_levels kwarg — the info is already carried by the EmbeddedOperator goal attached to the trajectory. Stretto was passing this kwarg in two call sites (compile_block and _compile_block_with_strategy), so any end-to-end run through the Piccolo API threw:

MethodError: no method matching SplinePulseProblem(...; ..., subsystem_levels=[3,3])
got unsupported keyword argument "subsystem_levels"

This was masked from CI because compile_block integration tests were sentinel-instrumented at commit a3a8e6e to never reach the solve. The failure surfaced once the Strettissimo v0.1 suite exercised the full pipeline end-to-end on a patched Piccolo.

Changes

• src/compile.jl — drop subsystem_levels = sys.subsystem_levels from both build_problem(...) call sites.

Test plan

• Validated via Strettissimo v0.1 integration suite (requires Piccolo PR #144 merged or dev-checked-out): 8/8 PASS, 5m46s.
• No other call sites reference the removed kwarg.
• Stretto's own CI should remain green (no behavior change for existing callers since the kwarg was simply being silently rejected, not consumed).

Dependencies

• Full pipeline validation requires Piccolo PR #144 (AbstractQuantumSystem parity for CompositeQuantumSystem) to be merged and released (or dev-checked-out). This Stretto-side change is independent and non-breaking on its own — merging it before Piccolo #144 doesn't change Stretto's observable behavior (the kwarg was never being consumed).

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