AP_AIS: guard truncated payload reads

[vlordier]

This draft isolates the AP_AIS payload bounds fix from the larger hardening branch.

Summary:

• reject truncated payload reads in AP_AIS bit extraction
• add focused regression coverage for truncated payload, last-valid-bit, and one-bit-past-end span handling

Why:

• this is a small, reviewable parser correctness fix
• it stands on its own without the broader security/process framing from the original branch

Validation:

• branch was split cleanly from upstream/master
• local configure succeeded in an isolated worktree
• the branch includes focused regression test source for the truncation and span-handling cases in this slice
• the target was not enumerated by the local worktree build as-is, so full test execution was not completed locally

This PR is intentionally draft while confirming the preferred test integration path.