Add a graduated bonding curve that switches steepness at configurable supply milestones

[Chucks1093]

Summary

The current bonding curve has a single exponent applied across the entire supply range. A graduated curve that applies a steeper exponent above configurable supply milestones would allow creators to design a pricing curve that starts accessible and becomes more exclusive at higher supply levels.

Scope

• Add configure_graduated_curve(key_id: BytesN<32>, milestones: Vec<(u32, u32)>) callable by the key creator
• Each milestone is a (supply_threshold, exponent) pair; exponents must be between 1 and 5
• In the buy price calculation, select the exponent corresponding to the current supply level
• Apply the milestone config before any keys are sold; panic with CurveAlreadyActive if sales have started
• Emit a graduated_curve_configured event with all milestone pairs

Acceptance Criteria

• Correct exponent applied at each supply tier
• Buy at supply below the first milestone uses the base exponent
• Exponent steps up correctly at each configured milestone threshold
• Configuration after sales have started panics with CurveAlreadyActive
• graduated_curve_configured event emitted with all pairs

ETA: 24 hours

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[TheShnider]

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[bonatkenneth-web]

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[ayomustap]

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[Richardkingz2019]

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[Daveeee-tech]

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[cythecode]

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