fix: match Costco mixed-tender card charges

[eshaffer321]

Summary

• match Costco warehouse receipts against bank-card tender totals when a receipt is paid with mixed tenders
• proportionally allocate item prices, subtotal, and tax to the card-paid amount for accurate split creation
• add a regression test and document the production bug in docs/bug-fixes.md

Verification

• golangci-lint run --timeout=5m
• go mod verify
• go mod tidy && git diff --exit-code go.mod go.sum
• go run golang.org/x/vuln/cmd/govulncheck@v1.3.0 ./...
• go test -v -race -coverprofile=coverage.txt -covermode=atomic ./...
• go build -v -o /tmp/itemize-ci-check ./cmd/itemize/
• temporary-db dry-run for receipt 21134300601232605091211 matched Monarch transaction 243571290918193554 at $209.65

Notes

• The May 28 $168.81 receipt still depends on Monarch receiving that transaction; it was not present when checked.

[codecov]

Codecov Report

❌ Patch coverage is 90.24390% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.54%. Comparing base (710f851) to head (db31604).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/adapters/providers/costco/provider.go	90.24%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #25      +/-   ##
==========================================
+ Coverage   56.29%   56.54%   +0.24%     
==========================================
  Files          43       43              
  Lines        4794     4830      +36     
==========================================
+ Hits         2699     2731      +32     
- Misses       1931     1933       +2     
- Partials      164      166       +2     

Flag	Coverage Δ
unittests	56.54% <90.24%> (+0.24%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
internal/adapters/providers/costco/provider.go	48.05% <90.24%> (+8.64%)	⬆️

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