Harden kink detection in Affine.price_elasticity

[alexanderthclark]

Motivation

• Avoid brittle float-equality when detecting kink points so that true kinks reliably raise an error while nearby floating-point-noisy inputs do not.
• Make kink detection configurable via tolerances so callers can control sensitivity for FP edge cases.

Description

• Replaced exact pt == self.intersections matching with np.isclose checks on both price and quantity coordinates against self.intersections, converting intersections to np.asarray for safe checks.
• Exposed tolerance parameters on Affine.price_elasticity as atol and rtol with sensible defaults 1e-12 and 1e-9, and threaded them through recursive above/below probes so diagnostics use the same tolerances.
• Added shape/size guards when inspecting self.intersections so detection only runs for properly-shaped intersection arrays.
• Added regression tests in tests/test_curves.py (TestAffinePriceElasticityKinks) covering exact-kink input (should raise), near-kink input (should not raise), and a floating-point representation edge case that should still be treated as a kink.

Testing

• Ran pytest -q tests/test_curves.py -k kink and observed 3 passed for the kink-focused tests.
• Ran pytest -q tests/test_curves.py and observed 32 passed indicating the module-level test suite succeeded.

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Codex Task