fix: UB in movement path advancement

[Belonit]

Summary

Fix invalid movement directions produced when locomotors advance an active path
with an overlapping memcpy.

Expected result: locomotors preserve every remaining path step and its
FACING_NONE terminator, so valid paths cannot expose uninitialized suffix
entries during movement.

The four fixes are:

• Centralize path advancement in FootClass::Advance_Path and use memmove
  for the overlapping shift. Initialize the complete path in the constructor
  and restore a canonical FACING_NONE suffix after every advancement.
• Validate generated-path offsets and lengths before copying them into the
  object path, and clear the regenerated suffix so failed or shorter path
  searches cannot expose stale directions.
• Stop stationary-occupier traversal at FACING_COUNT, which marks tunnel
  movement and is not a direction accepted by Adjacent_Cell.
• Check the path index before reading it during collision avoidance, preventing
  a separate one-past-end read when a terminator is missing.

Behavior and compatibility

Bug fix. Valid movement paths retain their existing behavior; path advancement
no longer depends on undefined overlapping-copy behavior. Maps, rules, saved
games, replays, network formats, deterministic simulation, and COM/ABI surfaces
are unchanged. No migration is required.

Validation

• An optimized clang-cl reproducer corrupted the sentinel with overlapping
  memcpy; changing only the operation to memmove preserved it.
• A sweep across active path lengths reproduced lost sentinels at several
  lengths with memcpy and none with memmove.
• The affected foot.cpp translation unit compiled successfully in the
  experimental clang-cl Release configuration.
• The supported Visual Studio 2022 Win32 Debug and Release builds were not run.

Documentation

No documentation change is needed. These are internal movement correctness
fixes that preserve the documented behavior and data formats.

Checklist

• The change is focused; unrelated mechanical cleanup is separate
• Compatibility effects and any migration are explicit
• Documentation impact is stated
• Validation distinguishes what passed, failed, and was not run
• No prohibited assets, binaries, SDKs, credentials, or generated output are included

[Belonit]

Updated after identifying the root cause: overlapping memcpy calls corrupted movement paths while advancing them. The fix now uses overlap-safe path advancement instead of recovering from invalid directions after corruption.

[github-actions]

Development builds of b6a391e:

• opents-Release-dd2b179
• opents-Debug-dd2b179

The links work without a GitHub account. Artifacts expire after 90 days, and this comment follows the latest successful build.