docs: add contract README and /// doc comments to aid_escrow public f…

[YahKazo]

app/onchain/contracts/aid_escrow/README.md — created from scratch with:

• What the contract does and the pool-based funding model
• ASCII state machine diagram (Created → Claimed / Cancelled → Refunded / Expired → Refunded)
• Full public function reference table with argument types, descriptions, and error lists
• Complete Error enum reference table (all 14 variants with codes and trigger conditions)
• Events table (all 11 event types with key fields)
• Build/test/deploy quick-start commands

src/lib.rs — /// doc comments added to all 21 public functions:

• Every function documents its # Arguments (with type and description) and # Errors (with the exact Error variant and the
  condition that triggers it)
• Replaced the old inline prose comments on revoke, cancel_package, extend_expiration, and withdraw_surplus with proper
  structured doc comments
• No logic was touched — only documentation
  closes Improve Contract README and Function Docs #74

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[Cedarich]

please resolve conflict

[Cedarich]

@YahKazo