[BUG-005] Populate buyer escrow index on fund

[isaacCodes1]

Overview

This PR fixes BUG-005 in contracts/escrow: when an escrow is created without a buyer and later funded by a signer, the buyer index was not always populated correctly. The fix ensures fund_escrow records the actual funding address in the buyer index after the token transfer succeeds, so get_escrows_by_buyer returns funded escrows for buyers who were not known at creation time.

Related Issue

Closes #150

Changes

Buyer Index Fix

• [MODIFY] contracts/escrow/src/lib.rs
• Added a small helper to append escrow IDs to the buyer index.
• Updated fund_escrow to always index the actual funding address after a successful token transfer.
• Updated get_escrows_by_buyer to read the persisted buyer index first and fall back to a scan only if the index is missing.

Regression Test

• [ADD] contracts/escrow/src/test_get_escrows_by_buyer.rs
• Added test_buyer_index_populated_on_fund to cover the case where the escrow is created with buyer = None and later funded.
• Kept the existing buyer lookup coverage intact.

Snapshot Updates

• [ADD] contracts/escrow/test_snapshots/test_get_escrows_by_buyer/test_buyer_index_populated_on_fund.1.json
• [MODIFY] contracts/escrow/test_snapshots/test_get_escrows_by_buyer/test_get_escrows_by_buyer.1.json
• Captures the buyer index writes produced by the updated logic.

Verification Results

Acceptance Criteria	Status
Buyer index is populated when escrow is funded by a previously unknown buyer	✅
get_escrows_by_buyer returns the funded escrow for that buyer	✅
Existing buyer lookup coverage still passes	✅
Targeted buyer-index tests pass	✅

How to Test

# Confirm the branch
git branch --show-current

# Run the buyer lookup regression tests
cargo test -p trustlink-escrow test_get_escrows_by_buyer -- --nocapture

# Run the full library test suite
cargo test -p trustlink-escrow --lib

Screenshots

✅ Buyer lookup regression passes

Run:

cargo test -p trustlink-escrow test_get_escrows_by_buyer -- --nocapture

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