Add compute_control_result: the control together with the solver status

[baggepinnen]

Closes #115, following the entry point suggested there.

result = compute_control_result(mpc, x; r, d, uprev, p)
result.control      # the control action (the solver's last iterate on failure)
result.exitflag     # >0 success, <0 failure
result.status       # the flag as a Symbol, e.g. :Optimal, :Soft_Optimal, :Primal_Infeasible
result.solver_info  # DAQP's info (iterations, solve time, ...)

• MPCResult{I} is a plain struct (parametric on the info type so it stays type-stable);
  compute_control_result performs exactly the same solve and uprev bookkeeping as
  compute_control, which now delegates to it (check = true/false behave as before).
• Implemented for MPC; ExplicitMPC's tree evaluation has no solver status, so it is left out —
  happy to add a trivially-:Optimal method if you prefer symmetry.
• Docstrings, a paragraph in manual/simple.md, and a testset (feasible: same control as
  compute_control on a fresh controller, :Optimal, info populated; infeasible hard output
  bound: exitflag < 1, :Primal_Infeasible, no throw, while compute_control still asserts).

One observation from writing the tests, possibly worth its own issue: after an infeasible solve,
subsequent solve calls on the same mpc.opt_model return exitflag = 1 with NaN control —
the Julia-side DAQP workspace is not reset after a failure (the generated C resets it every call).
The tests therefore use a fresh controller per assertion. That behaviour makes the status all the
more important to surface: the second call reports :Optimal while returning NaN.

Full test suite passes locally (Julia 1.12.7).

🤖 Generated with Claude Code

https://claude.ai/code/session_01LNTSoj8uSmNpSqpRTor8di

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✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #117      +/-   ##
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+ Coverage   89.57%   89.59%   +0.01%     
==========================================
  Files          15       15              
  Lines        2389     2393       +4     
==========================================
+ Hits         2140     2144       +4     
  Misses        249      249              

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