Add an end-to-end OLS tutorial

[timholy]

A small ordinary-least-squares library wrapped end-to-end as a worked example for the docs: write the Julia source, run build_library to compile and generate wrappers, pip install the resulting package into a clean virtualenv, and call it from Python. The example deliberately covers the full recognized JLWInterop vocabulary in one library — CMatrix{Float64} for the design matrix, CVector{Float64} for response / coefficients / predictions, CString for a report buffer, JLWStatus both as a direct return (predict) and embedded in a result struct (fit's FitResult), and a scalar r_squared.

Lives at examples/ols/; new tutorial page docs/src/tutorial.md sits between Home and Concepts in the docs nav so it acts as the on-ramp.

Known blocker: the example builds end-to-end (build_library succeeds and produces an importable Python package), but the compiled .so cannot yet run its fit entrypoint from a host process. X \ y reaches BLAS through Base.Libc.Libdl.LazyLibrary, whose untyped fields and runtime-hook (jl_libdl_dlopen_func) dispatch put the cold load path outside what juliac --trim=safe's static call-graph analysis can keep alive — the first BLAS call from Python aborts with MethodError: no method matching Base.Libc.Libdl.dlopen(::LazyLibrary, ::UInt32). The tutorial is written as if that works (and once the fix lands it will, with no further changes here); merging it now documents the intended workflow and gives the JLW release docs something to point at.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.66%. Comparing base (411f734) to head (acabec8).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/build.jl	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #42      +/-   ##
==========================================
- Coverage   95.01%   94.66%   -0.35%     
==========================================
  Files           5        5              
  Lines         822      825       +3     
==========================================
  Hits          781      781              
- Misses         41       44       +3     

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