Add multivariate optimization

[nyo16]

This PR adds two multivariate optimization algorithms to Scholar, completing the optimization module:

• BFGS - Quasi-Newton method with automatic differentiation for smooth, differentiable functions
• Nelder-Mead - Derivative-free simplex method for functions where gradients are unavailable or expensive

Both implementations follow the patterns established in PRs #327 (Golden Section) and #328 (Brent):

• defn entry point with deftransformp for option validation
• Initial point as explicit function argument
• Module constants used directly (not wrapped in Nx.tensor)
• Each module has its own NimbleOptions schema
• JIT/GPU compatible

Changes

File	Description
lib/scholar/optimize/bfgs.ex	BFGS implementation with backtracking line search
lib/scholar/optimize/nelder_mead.ex	Nelder-Mead simplex implementation
test/scholar/optimize/bfgs_test.exs	Tests for BFGS (sphere, Rosenbrock, Booth, Beale functions)
test/scholar/optimize/nelder_mead_test.exs	Tests for Nelder-Mead
notebooks/optimize.livemd	Updated with multivariate optimization section
notebooks/efficient_frontier.livemd	Portfolio optimization example using both algorithms
compare_*.py	SciPy validation scripts

Test plan

• All existing tests pass (mix test)
• New algorithm tests pass (mix test test/scholar/optimize/)
• Doctests pass
• Code formatted (mix format)
• Test livebooks manually in Livebook"

[nyo16]

And comparing them with python

BFGS Results:
  ┌───────────────────┬──────────────┬────────────────────┬────────┐
  │     Function      │ Expected Min │    SciPy Found     │ Status │
  ├───────────────────┼──────────────┼────────────────────┼────────┤
  │ sphere_2d         │ [0, 0]       │ [~0, ~0]           │ ✓      │
  ├───────────────────┼──────────────┼────────────────────┼────────┤
  │ sphere_3d         │ [0, 0, 0]    │ [~0, ~0, ~0]       │ ✓      │
  ├───────────────────┼──────────────┼────────────────────┼────────┤
  │ rosenbrock        │ [1, 1]       │ [0.99999, 0.99999] │ ✓      │
  ├───────────────────┼──────────────┼────────────────────┼────────┤
  │ booth             │ [1, 3]       │ [1, 3]             │ ✓      │
  ├───────────────────┼──────────────┼────────────────────┼────────┤
  │ beale             │ [3, 0.5]     │ [3, 0.5]           │ ✓      │
  ├───────────────────┼──────────────┼────────────────────┼────────┤
  │ shifted_quadratic │ [2, -3, 1]   │ [2, -3, 1]         │ ✓      │
  └───────────────────┴──────────────┴────────────────────┴────────┘
  Nelder-Mead Results:
  ┌───────────────────┬──────────────┬──────────────┬────────┐
  │     Function      │ Expected Min │ SciPy Found  │ Status │
  ├───────────────────┼──────────────┼──────────────┼────────┤
  │ sphere_2d         │ [0, 0]       │ [~0, ~0]     │ ✓      │
  ├───────────────────┼──────────────┼──────────────┼────────┤
  │ sphere_3d         │ [0, 0, 0]    │ [~0, ~0, ~0] │ ✓      │
  ├───────────────────┼──────────────┼──────────────┼────────┤
  │ rosenbrock        │ [1, 1]       │ [1, 1]       │ ✓      │
  ├───────────────────┼──────────────┼──────────────┼────────┤
  │ booth             │ [1, 3]       │ [1, 3]       │ ✓      │
  ├───────────────────┼──────────────┼──────────────┼────────┤
  │ beale             │ [3, 0.5]     │ [3, 0.5]     │ ✓      │
  ├───────────────────┼──────────────┼──────────────┼────────┤
  │ shifted_quadratic │ [2, -3, 1]   │ [2, -3, 1]   │ ✓      │
  └───────────────────┴──────────────┴──────────────┴────────┘