fix(parse): reject incomplete format matches - #152

Thanks for the PR! I agree this is a bug. I'll handle docs/ and skills/ myself after the merge.

A few changes before merging:

1. Rename _error to _mismatch. preparse() doesn't decide validity. The flag records a matching result, like _match.

2. Set the flag in one place. Wrap the loop in an inner function that returns whether the whole pattern was matched: each break becomes return false, except at ..., which returns true. This way, a branch added later can't forget to set it.

3. Keep the old _index for a wildcard past the end of the input. With the str.length > 0 guard, preparse('2025-01-01', 'YYYY-MM-DD ') returns _index: 10, _length: 10. On master it returns 11, 10, so code that checks _index === _length itself would start accepting this input. Advance _index as before, then stop the loop.

   const matchPattern = () => {
     for (const token of pattern) {
       // ...
       } else if (token === str[0] || token === wildcard) {
         pr._index++;
         if (!str) {
           return false;
         }
       } // ...
     }
     return true;
   };
   
   if (!matchPattern()) {
     pr._mismatch = true;
   }

   I checked this sketch against all 19 cases in your test file, and they all give the results you expect.

4. Move the tests into tests/preparse.spec.ts, tests/isValid.spec.ts, and tests/parse.spec.ts. Test files mirror src/.

One note: this isn't quite free of breaking changes. Input that used to parse, such as parse('2025-01', 'YYYY-MM-DD'), now returns Invalid Date. No need to update the description, though.