feat(v2): KIX, the second four-state postal code

[s2x]

KIX — the Dutch four-state code, kix, aliases kix-code, klantindex, postnl. Pure PHP, no options.

What it is

RM4SCC's alphabet with the envelope taken off. The whole symbol:

C1 C1 C1 C1  C2 C2 C2 C2  ...  Cn Cn Cn Cn

No start bar, no stop bar, no check character. So KixTest asserts the symbology in one line — a payload's bars are its characters' bars concatenated — because every mistake available here is a mistake of addition.

The shared alphabet

The thirty-six characters are identical to RM4SCC's. That was measured, not assumed: the two fixtures were drawn independently through zint and agree character for character. So the derivation moved into Generator\FourState\Alphabet and Rm4scc\Characters now holds only what is RM4SCC's — start bar, stop bar, check character. Two copies of a base-six derivation are two chances for one to drift, and a drifted copy still draws thirty-six legal characters; a test asserts they still agree.

What KIX does not do

Worth stating, because a postal code sounds like something that checks itself:

• No error detection over the symbol. A dropped or added character produces another legal KIX symbol saying something else. What survives is the per-character two-of-four parity, which catches a misread bar, not a misread symbol. Sorting machines read KIX beside an address they can compare it against.
• No start or stop pattern. The quiet zone is load-bearing here in a way it is not in RM4SCC — with nothing to recognise, white space is the only thing marking where the first character begins — so it stays on all four sides.

Eighteen characters is the ceiling, and this time it is the specification's rather than the reference encoder's.

Verification

Same arrangement as RM4SCC, same caveat: no free decoder reads a four-state postal code, so this is one independent opinion where the rest of the library has two.

• tools/kix_reference.py → tests/fixtures/kix_reference.csv, 188 symbols, 36/36 characters, lengths 1 to 18. Every character is drawn alone, at the front, at the back and mid-symbol, since an encoder that had grown an RM4SCC-shaped start bar would still get every one-character symbol right. The tool asserts zint's own output is a concatenation before writing anything.
• The substituted round trip — the rendered PNG measured back into state letters and compared with zint's — is now one test over the family rather than one per symbology. The next four-state code adds a row to fourStateProvider() instead of deciding for itself whether it wanted the gate. Same for the expiry test that fails the day zxing-cpp learns to read one.

The encoder matched zint on 2500GG30250 on the first run and on all 188 fixture rows.

Checks

composer lint clean; full suite green (15390 tests, 264323 assertions, the 21 pre-existing PHPUnit deprecations); SCANME_REQUIRE_DECODER=1 composer test:roundtrip green (356 tests, 1389 assertions).