Add TransactionValidator and shared SubunitConverter (R2.1)

[jules-paystack]

Summary

• Adds Gateway/Validator/TransactionValidator.php: pure checks of a Paystack verify-transaction response against status, currency, a bounded amount window, and live/test domain — not wired into any of the three payment consumers yet.
• Adds Gateway/SubunitConverter.php, a shared static helper repointing Controller/Payment/Setup.php's previously-inline subunit computation so the send side and this validator can never independently drift.
• Adds PaystackApiClient::isTestMode() as the single source of truth for test_mode, so a future caller doesn't introduce a second independent config read.
• Went through a full plan → 3-critic panel → implement → 2-critic diff review → fix loop → dedicated test pass cycle (docs/plans/2026-09-28-r2-1-transaction-validator.md, untracked per this repo's convention for unfixed money-path defect maps).
• Before implementing the amount check, measured on dev-repro/ that the browser's popup amount and the server's post-placement order total agree exactly, on both a same-currency and a display≠base-currency guest checkout — the parent plan's explicit blocking prerequisite for this item.

Test plan

• Test/Unit/vendor/bin/phpunit -c phpunit.xml --no-coverage — 162 tests, 225 assertions, all green
• Independent Gate 3 test-runner pass found and closed 2 real coverage gaps beyond the implementer's own tests
• Adversarial security-critic + architecture-critic review of both the plan and the final diff, with a documented fix loop (3 high-severity findings actioned: an in-flight-vs-failed status regression against the shipped D5 fix, a "did money move" ambiguity on the domain-mismatch code now resolved via chargeIsReal(), and a duplicated test_mode config read)
• Runtime measurement on dev-repro/ (2.4.9, PHP 8.5, CSP on) confirming the amount comparand is correct before it was implemented

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