Four start-of-input checks in the main parse loop still used !last
(falsy NUL) to detect beginning-of-input. After the TOKENIZER_EOF
sentinel change, Tokenizer_read_backwards returns 0x110000 instead of
'\0', so !last is always false and headings, lists, and horizontal
rules at position 0 would silently fail to parse.

The C tokenizer was silently truncating input at the first NUL byte (\x00)
because it used '\0' both as a valid input character and as the EOF sentinel.

Root cause:
- Tokenizer_read() returned '\0' for both:
  1. Real NUL bytes in the input
  2. End-of-input (when index >= text.length)
- This made them indistinguishable, causing real NULs to be treated as EOF

Fix:
1. Define TOKENIZER_EOF as 0x110000 (first invalid Unicode code point)
2. Update Tokenizer_read() and Tokenizer_read_backwards() to return
   TOKENIZER_EOF instead of '\0' for out-of-bounds reads
3. Replace all `!this` and `'\0'` checks with explicit `TOKENIZER_EOF` checks
4. Remove '\0' from the MARKERS array (no longer needed as EOF marker)
5. Move EOF check before is_marker() in main parse loop to ensure
   TOKENIZER_EOF doesn't try to emit as a character
6. Fix Tokenizer_has_leading_whitespace() to recognize TOKENIZER_EOF

The Python tokenizer already preserved NUL bytes correctly; this brings
the C tokenizer into parity.

Regression test added: test_nul_byte_preservation() verifies that both
tokenizers now preserve NUL bytes in plain text, templates, and
multiple-NUL scenarios.

Bumps [pytest](https://github.com/pytest-dev/pytest) from 9.0.2 to 9.0.3.
- [Release notes](https://github.com/pytest-dev/pytest/releases)
- [Changelog](https://github.com/pytest-dev/pytest/blob/main/CHANGELOG.rst)
- [Commits](https://github.com/pytest-dev/pytest/compare/9.0.2...9.0.3)

---
updated-dependencies:
- dependency-name: pytest
  dependency-version: 9.0.3
  dependency-type: direct:development
...

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Bumps [pygments](https://github.com/pygments/pygments) from 2.19.2 to 2.20.0.
- [Release notes](https://github.com/pygments/pygments/releases)
- [Changelog](https://github.com/pygments/pygments/blob/master/CHANGES)
- [Commits](https://github.com/pygments/pygments/compare/2.19.2...2.20.0)

---
updated-dependencies:
- dependency-name: pygments
  dependency-version: 2.20.0
  dependency-type: indirect
...

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Bumps [requests](https://github.com/psf/requests) from 2.32.5 to 2.33.0.
- [Release notes](https://github.com/psf/requests/releases)
- [Changelog](https://github.com/psf/requests/blob/main/HISTORY.md)
- [Commits](https://github.com/psf/requests/compare/v2.32.5...v2.33.0)

---
updated-dependencies:
- dependency-name: requests
  dependency-version: 2.33.0
  dependency-type: indirect
...

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