Implement complete RFC 8259 number format parsing and tests

[ionux]

The old number scanner was a single while-loop over digits and '.'. It silently accepted illegal inputs (lone '-', leading zeros like '012') and caused OKJ_ERROR_SYNTAX on valid exponent notation like '1e10'.

Replace with an explicit four-step grammar walk that matches RFC 8259 §6:
number = [ minus ] int [ frac ] [ exp ]

Changes in src/ok_json.c:

• Step 1 (minus): consume '-' only when a digit follows; reject bare '-' with OKJ_ERROR_BAD_NUMBER
• Step 2 (int): accept '0' only when NOT followed by another digit (leading-zero rule); otherwise consume digit1-9 *DIGIT
• Step 3 (frac): if '.' present, require and consume 1+ digits after it
• Step 4 (exp): if 'e'/'E' present, consume optional sign then require and consume 1+ digits; reject 'e' or 'e+' with no digits

Valid formats now accepted: 0, -0, 42, -42, 3.14, -1.5, 1e10,
2.5E-3, 1E+2, 0.5e2

Invalid formats now rejected: '-', '012', '1.', '1e', '1e+'

Changes in test/ok_json_tests.c — 8 new tests:

• test_number_negative: -42 accepted, length=3
• test_number_float: 3.14 and -1.5 accepted with correct lengths
• test_number_exponent: 1e10, 2.5E-3, 1E+2 all accepted
• test_number_zero_variants: 0 (length=1) and -0 (length=2) accepted
• test_number_invalid_lone_minus: '-' alone -> OKJ_ERROR_BAD_NUMBER
• test_number_invalid_leading_zero: '012' -> OKJ_ERROR_BAD_NUMBER
• test_number_invalid_trailing_decimal: '1.' -> OKJ_ERROR_BAD_NUMBER
• test_number_invalid_exponent_no_digits: '1e' and '1e+' -> OKJ_ERROR_BAD_NUMBER

All 40 tests pass under -Wall -Wextra -Werror -pedantic -std=c99.

https://claude.ai/code/session_01UZpNhYhgmEd893sSpBNi6P