be: cover the czech text, the holder's delegates and the fallbacks, so a regression fails the build - #74

Closes #73. Tests only; every one was checked against the code it guards by breaking that code and watching the test fail.

• Czech text: the pending fixture's Okamžitá platba is now asserted in message(), and the booked fixture's OKAMŽITÁ ÚHRADA in raw(). Both fail when the test server answers with charset=ISO-8859-1.
• The holder's delegates: accounts(), balance(id) and transactions(id) are called and must reach the paths they name; balance() on a two-account token is IllegalStateException. Both fail when balance(id) reads onlyAccountId() or accounts() returns nothing.
• Kotlin: me.accountByIban("…")?.id ?: … compiles and me.accountByIban("…").id does not, beside the existing JSpecify checks.
• Fallbacks: an unknown clearingSystemReference is Method.OTHER with the code kept in raw(), and reversalIndicator: true gives reversed().
• Edges: Money.negate(), a currency without minor digits (XXX, the Math.max(…, 0) branch), and a null IBAN refused before any request.

From the edge list, the IBAN dash and mod-97 points were settled in #72: a dash is dropped with every separator and a failing check digit is an IllegalArgumentException, both tested there. No Kover: worth it only if coverage should be answered mechanically next time.