test: add golden-file test to prevent accidental fixture removal

[martinshub-tech]

Closes #35

Summary

This PR introduces a golden-file validation check to scripts/validate-fixtures.mjs to prevent accidental removal or shrinkage of fixture sets (e.g. during a bad merge resolution).

• Added scripts/__fixtures__/expected-counts.json: Stores the minimum counts (total, clean, suspicious, malicious) and the explicit list of the 11 original seed fixture IDs.
• Modified scripts/validate-fixtures.mjs: Reads expected-counts.json and asserts that the current fixture state is at or above the expected minimums, and that all 11 original seed fixtures are present.

Type of change

• Fixture change — adds, removes, or rescores a destination or asset
• Transaction change — adds or removes a sample XDR
• Documentation only — README, CONTRIBUTING, CHANGELOG prose, etc.
• CI / tooling

Changelog

• N/A — CI/tooling change only.

Score-regression guard

• N/A — no scores were changed.

Validation

• npm run validate passes locally.