Fixed libais CVE-2026-56770

 This is a weird milestone: libais got its first CVE. CVE-2026-56770 is a crash coming from my 03f1c92418 where I did "vdm: int → size_t for sequence and sentence numbers" to avoid comparing signed and unsigned numbers and I was able to get rid of "-Wno-sign-compare". I added a "kNoSequenceNumber" that has a huge value (for AIS sequence numbers) of 999999. But, I have code checking the sequence numbers that was only asserts and I was counting on the old value of -1.

So the bug has only been around since Sep 2025 and I haven't done a libais release in more than a decade (ahem, I need to do a release). So it shouldn't impact too many people. According to Gemini, libais falls under "Minor Historic and Niche Libraries" so... ouch.

This patch should fix the issue: ce0fedeba5 / libais#264

libais needs so much cleanup at this point that it is painful. I also asked Antigravity to do a scan of the code for troubles. In addition to finding lots of spelling mistakes, there are plenty of places to improve the code. Sigh.

Here is a sample from ais8.cpp:

### Specific Typos and Grammatical Mistakes
* **Line 293**: 
  ```cpp
  // Some people transmit without the idiodic spare padding
  ```
  *Typo*: `idiodic` should be spelled **`idiotic`**.
* **Line 138**: 
  ```cpp
  // TODO(schwehr): how to treat this spare vrs water level?
  ```
  *Typo*: `vrs` should be spelled **`vs`** or **`versus`**.
* **Lines 324 & 358**: 
  ```cpp
  horz_viz(0.0) ... horz_viz = bits.ToUnsignedInt(246, 8) / 10.;
  ```
  *Inconsistency/Typo*: Abbreviating "horizontal visibility" as `horz_viz` uses a 'z' instead of 's' (`horz_vis`), which conflicts with line 72 and line 
568 where it is correctly named `horz_vis`.
* **Line 421**: 
  ```cpp
  ice_thickness = bits.ToUnsignedInt(324, 7) / 100.;  // m.  Network is cm,
  ```
  *Grammar*: The inline comment ends abruptly with a trailing comma (`cm,`) instead of completing the sentence or ending with a period.

In addition to mispelling "idiotic," I can just remove that word. I don't really need to express my frustration over the AIS specification like that. 😎

Thanks to github.com/raefko for the bug report!