feat(invoice-escrow): add custom fee schedule per invoice category

[richardgittest]

Summary

Implements custom fee schedule per invoice category in contracts/invoice-escrow.

Closes the linked issue.

Changes

types.rs

• Added InvoiceCategory enum: Standard, Factoring, Reverse, Government
• Added CategoryFeeSchedule { fee_bps: u32 } struct
• Added StorageKey::CategoryFee(InvoiceCategory) variant
• Added category: InvoiceCategory and effective_fee_bps: u32 fields to EscrowData

errors.rs

• Fixed pre-existing duplicate discriminant bug (InvalidPayer was = 15, same as Paused) — bumped to 16, InvalidDueDate to 17
• Added CategoryFeeNotFound = 19

storage.rs

• Added get_category_fee, set_category_fee, remove_category_fee helpers (instance storage)
• Merged upstream TTL management and buyer whitelist helpers

events.rs

• Added category_fee_set(category, old_fee_bps, new_fee_bps) event
• Extended escrow_created signature with category (as u32) and effective_fee_bps
• Merged upstream escrow_status_changed event

lib.rs

• set_category_fee — admin-only, validates fee_bps ≤ 10_000, emits cat_fee_set
• get_category_fee_schedule — view returning schedule or CategoryFeeNotFound
• create_escrow — accepts new category: InvoiceCategory param; resolves and stamps effective_fee_bps (category override > global default)
• record_payment — uses data.effective_fee_bps (immutable, stamped at creation)
• Merged upstream whitelist enforcement in fund_escrow and escrow_status_changed calls

test.rs

• All 54 existing create_escrow call-sites updated with new category param
• Event tuple assertions updated to reflect new 11-field escrow_created data
• 19 new tests added covering: set_category_fee (success, auth, invalid bps, max/zero, overwrite, event), get_category_fee_schedule (not found, independence), fee resolution (global fallback, override, precedence, all 4 categories), record_payment with category fee (immutability after creation, zero-fee), event field verification

What was tested

• All 54 existing create_escrow call-sites compile with the new category parameter
• 19 new unit tests cover every new code path and edge case
• Merged cleanly with upstream/dev (whitelist feature + TTL management)

Closes #76

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[chizzy192]

@richardgittest
FIX MERGE CONFLICTS PLS

[chizzy192]

Hi @richardgittest! 👋 Thanks for your PR. CI checks are failing (2 failing checks). Please check the test failures and update your branch.

[chizzy192]

Hi @richardgittest, please rebase on dev to resolve conflicts in contracts/invoice-escrow/src/fee.rs. Verify that custom fee category calculations handle boundary fee basis points (0 BPS and 10,000 BPS max) cleanly in cargo test.