Add configurable fee with linear decay to bid wall

[pileks]

This PR introduces a configurable fee mechanism with time-based linear decay for the bid wall contract. Instead of a hardcoded 1% fee, bid walls can now be initialized with a maximum fee, minimum fee, and decay duration. The fee starts at max_fee_bps when the bid wall is created and linearly decays to min_fee_bps over fee_decay_duration_seconds. This creates an incentive structure that discourages early selling while providing a steady-state fee floor.

Changes

State Extension (bid_wall.rs)

• Added three new fields to the BidWall account:
  • fee_decay_duration_seconds: u32 - duration over which the fee decays
  • max_fee_bps: u16 - starting fee in basis points
  • min_fee_bps: u16 - floor fee in basis points

Fee Calculation (sell_tokens.rs)

• Removed the hardcoded FEE_BPS constant (was 100 bps / 1%)
• Implemented linear fee decay formula: max_fee - (max_fee - min_fee) * bid_wall_age / fee_decay_duration
• Fee is clamped to min_fee_bps after the decay period completes
• Uses u128 arithmetic to prevent overflow during intermediate calculations

Initialization (initialize_bid_wall.rs)

• Added validation requiring fee_decay_duration_seconds > 0 to prevent division by zero
• Extended InitializeBidWallArgs with the three new fee parameters
• Fee configuration is emitted in BidWallInitializedEvent

Launchpad Integration (complete_launch.rs)

• Extracted PASS_THRESHOLD_BPS constant (300 bps / 3%)
• Default bid wall configuration for launches:
  • fee_decay_duration_seconds: 14 days
  • max_fee_bps: 500 (5%, which is pass threshold + 200 bps)
  • min_fee_bps: 300 (3%, equal to pass threshold)

SDK Updates

• Extended BidWallClient.initializeBidWallIx() with required fee parameters
• Updated TypeScript IDL types for the new state fields and events

Mechanics

The fee decay mechanism works as follows:

1. At t=0 (bid wall creation), sellers pay max_fee_bps
2. The fee decreases linearly over fee_decay_duration_seconds
3. After the decay period, the fee stabilizes at min_fee_bps

The formula ensures the fee is always at least min_fee_bps via .max():

let fee_bps = min_fee_bps.max(
    max_fee_bps - (max_fee_bps - min_fee_bps) * bid_wall_age / fee_decay_duration
);

For the default launch configuration (5% -> 3% over 14 days).

Remaining work

• Add bid wall account migration (or cut a new release) if these changes are accepted.
  • Existing bid walls can have the following parameters:
  • fee_decay_duration_seconds = 1
  • max_fee_bps = 100
  • min_fee_bps = 100