feat(rwa): add comprehensive validation, authority checks, and event emission in all instructions

[syed-ghufran-hassan]

Security hardening: replaced opaque process() delegations with inline validation in every instruction.

• Added require! checks for:
  • Non‑zero amounts / supplies
  • Authority (admin / owner) on all write operations
  • Asset existence and state transitions (not frozen, not already tokenized)
  • Metadata length limits and non‑empty strings
  • Mint and token account consistency
  • Overflow protection using checked_sub
• Prevented immutable field updates (symbol, total_supply) during metadata updates
• Emitted structured events for all state mutations to aid off‑chain indexing
• Integrated custom error codes from errors.rs for precise failure reporting

This makes the contract auditable, safer against front‑running and misuse, and fully
self‑contained without relying on hidden logic in separate process implementations.