Add max_supply_updated event to set_max_supply in learn-token

[DeFiVC]

What

The set_max_supply function in learn-token does not emit an event when the supply cap is changed.

Why

Supply cap changes significantly affect tokenomics. Indexers, monitoring systems, and block explorers need events to detect when the cap changes. The related set_progress_tracker function already emits a progress_tracker_updated event — set_max_supply should follow the same pattern for consistency.

Scope

• Define a max_supply_updated event (or reuse existing patterns)
• Emit the event in set_max_supply with the old and new values
• Add a test verifying the event is emitted

Technical Context

• File: contracts/learn-token/src/lib.rs (lines ~453-464)
• set_progress_tracker already emits progress_tracker_updated — follow this pattern
• Events in Soroban are emitted via env.events().publish()

Acceptance Criteria

• set_max_supply emits an event with old and new values
• Event is testable and verified in unit tests
• Follows existing event patterns in the codebase

[odiliflash-ops]

I would love to work on this issue