test(credential): make the binding tests fail when binding is removed

[Jaro-c]

The four tests named for purpose and subject binding asserted nothing about binding. Each called Issue once per purpose or subject, so the two tokens were independent CSPRNG draws and the hashes differed because the tokens differed. Deleting writeField(mac, purpose) from computeHash left all four green.

test	with purpose dropped, before
TestIssue_DifferentPurposesDifferentHashes	pass
TestIssue_DifferentSubjectsDifferentHashes	pass
TestVerify_WrongPurposeRejected	pass
TestVerify_WrongSubjectRejected	pass

The control was defended, but by accident. The sabotage failed TestComputeHash_LengthPrefixSurvivesEmbeddedNULs and one seed of FuzzVerify, two tests about NUL handling and length prefixing that happen to hold purpose and subject fixed. Narrowing either one would have removed the only real coverage while every test still carrying the word "binding" stayed green.

What changed

The Issue tests now compare computeHash directly with the token held fixed, so exactly one field varies and a pass is evidence about that field.

The Verify tests are written as the attack they are named for: one real credential replayed into another flow. Same token, same stored hash, only the purpose or the subject different. Presenting a different token against a different hash, which is what they did, fails on the token alone and proves nothing.

Each Verify test now also asserts the credential verifies under its own purpose and subject before asserting the cross-flow rejection. Without that, a Verify that rejected everything would pass.

Measured after

Dropping purpose from computeHash fails the two purpose tests and leaves the subject tests green. Dropping subject does the reverse. Each test is now about the field it is named after, and the sabotages compiled in both directions.

Also removed a stray _ = time.Now() and its now-unused import.

Provenance

Found by a MiniMax-M3 pass over the package. It predicted both halves before anything was run: that all four tests would stay green, and that the NUL tests would catch the sabotage incidentally. Both held.

Closes #358