Add op_system metadata passthrough to op_engine connector

[jc-macdonald]

Forward op_system axis and operator metadata through the op_engine connector so that ModelCore and CoreSolver can use structured population dimensions and IMEX operator splitting.

Problem

Mixing kernels flow from op_system to op_engine (merged as extra params), but operator specs and axis-aware metadata are not forwarded. The engine constructs ModelCore with hardcoded n_subgroups=1 and ignores the system's axis structure entirely.

Scope

• Read axis metadata from system.option("axis_order"), system.option("axis_sizes"), etc. (or the formal hook once flepimop2 #131 lands)
• Build ModelCore using actual axis sizes: n_subgroups from axis meta, other_axes from additional dimensions
• Read operator specs from system.option("operators", None) and merge with config-level operator overrides into OperatorSpecs
• Set operator_axis from meta when available
• Validate operator shapes match axis lengths
• Keep existing mixing-kernel merge as fallback

Files

• flepimop2-op_engine/src/flepimop2/engine/op_engine/engine.py (run() method)
• flepimop2-op_engine/tests/

Blocked by

• op_system update flepimop2 provider pacakge #12 — system must expose metadata via options
• op_system Tighten op_engine & op_system integration through their connectors #8 — operator specs must be wired in op_system first
• op_engine Replace getattr introspection with system.option() for mixing kernels #42 — system.option() adoption

Related

• op_engine Implement validate_system() in flepimop2 provider #41 (validate_system) — validation should check axis/operator compatibility
• flepimop2 #131 — may standardize axis/kernel hooks on SystemABC

[jc-macdonald]

Superseded by PR #12 (provider refactor), which consolidated the provider package and aligned it with flepimop2's ModuleABC refactor (PR ACCIDDA/flepimop2#118). The metadata passthrough this issue described is now handled via system.option() — remaining work tracked in #42.