feat: standardize token event schemas and add Makefile preflight target

[Cedarich]

Summary

This PR implements two key improvements to the Veritix Pay contract:

1. Standardized Token Event Schemas - All core token operations now follow consistent event topic/payload conventions
2. Makefile Preflight Target - Fast validation of required developer tooling

Changes

Event Schema Standardization

Standardized event schemas across all token operations to improve indexer and downstream integration maintainability:

Core Token Events

• mint: (mint, admin, to) → amount
• burn: (burn, from) → amount
• burn_from: (burn, spender, from) → amount
• transfer: (transfer, from, to) → amount
• approve: (approve, from, spender) → amount
• clawback: (clawback, admin, from) → amount

Module Events

• escrow_created/released/refunded: Consistent (event_name, id, address) format
• dispute_opened/resolved: Simplified to single symbol identifiers
• recurring_setup/executed/cancelled: Standardized payload structure
• split_distributed: Single symbol with structured payload

All events now use symbol_short!() for the primary event identifier, following Soroban best practices and reducing gas costs.

Developer Tooling - Preflight Target

Added make preflight target to validate required tools before build:

make preflight  # verify all required tools are installed

Closes #100 
Closes #99 

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