feat: support alphanumeric CNPJ (July 2026 format)

[LeoColman]

What

Adds support for the alphanumeric CNPJ format, in effect since July 2026.

The first twelve characters may now be digits (0-9) or uppercase letters (A-Z); the last two remain numeric verification digits. Total length stays 14.

Algorithm

Each character is worth its ASCII code minus 48, so 0-9 map to 0-9 and A-Z continue the sequence from 17 (A) to 42 (Z). The module-11 weights are unchanged, which makes the change fully backwards compatible — every legacy numeric CNPJ still validates.

Verified against the official SERPRO / Receita Federal documentation, including its worked example 12.ABC.345/01DE → check digits 35.

Changes

• isCnpj() accepts A-Z in the first 12 positions; last 2 must be digits.
• Replaced Char.isDigit() with explicit '0'..'9' / 'A'..'Z' ranges — isDigit() accepts non-ASCII digits (e.g. Arabic-Indic ٦) that would corrupt the ASCII-48 math.
• Long.isCnpj() unchanged (numeric-only by nature; an alphanumeric CNPJ is not representable as a number).
• Lowercase letters rejected, per the spec (uppercase only).
• README documents the new format.

Tests

• Official Receita Federal example, raw and formatted.
• 8 valid alphanumeric vectors + the same 8 with corrupted check digits.
• New ValidNumericCnpjGenerator property test locking legacy numeric compatibility.
• Rejection cases: lowercase, letters in DV positions, non-ASCII digits, junk chars.

All 16 tests pass; detekt clean.