Support perfect-segment (horizontal/vertical) curve intersections in Equilibrium

[alexanderthclark]

Motivation

• Equilibrium previously rejected any demand or supply with perfectly elastic/​inelastic segments at construction (Equilibrium.__init__) which prevented valid intersections from being computed; the goal is to handle common perfect-segment combinations instead of blocking them.
• The change moves special-case handling into the intersection logic so Equilibrium._compute/_find_intersection can return a deterministic (p, q) when the geometry uniquely pins an outcome.
• When economic indeterminacy (price pinned but quantity not) or genuinely unsupported pairings occur, the model should raise clear, specific errors rather than a blanket rejection.

Description

• Removed the unconditional perfect-segment rejection from Equilibrium.__init__ so curves with perfect segments may be used.
• Added a helper Equilibrium._solve_with_perfect_segments(dcurve, scurve) that inspects piece combinations and handles: horizontal vs regular, vertical vs regular, and horizontal vs vertical cases, collecting candidate (p,q) solutions and returning a unique deterministic pair when possible.
• Wired the helper into Equilibrium._find_intersection to run before the regular piecewise intersection loop and preserved the affine fallback behavior for ordinary cases.
• Introduced distinct error messages for failure modes: "economically indeterminate quantity at fixed price" for price-equal horizontal pairs and "unsupported combination: ..." for other unhandled perfect-segment pairings or multiple distinct perfect-segment intersections.
• Made domain validation in _valid_in_domain orientation-safe by using min/max on piece domains so intersections validate correctly regardless of domain ordering.
• Updated tests in tests/test_horizontal_curves.py to reflect supported perfect-segment outcomes and the new indeterminate error case.

Testing

• Ran pytest -q tests/test_horizontal_curves.py tests/test_equilibrium.py tests/test_world_price.py and all tests passed (15 passed).
• The updated horizontal-curve tests exercise horizontal vs regular and horizontal vs horizontal indeterminacy and passed.

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Codex Task