Fix shroud stack overflow and the F4 playtest (un)shroud toggle

[torradmin]

Summary

Two related fixes to shroud/fog handling:

• DisplayClass::Fog_Cell regrew fog by recursing into every neighboring cell
  that also needed to regrow. Unshrouding or reshrouding a large connected
  area (a big map, a reveal-the-map crate) could recurse deep enough to
  overflow the stack and crash. It now walks an explicit queue instead.
• The F4 playtest shroud toggle only flagged the tactical view for redraw and
  never touched the underlying shroud data, so pressing it did nothing
  visible. It now reveals/reshrouds the map through the same path the
  reveal-the-map and blackout crates use, and MapClass::Is_Shrouded now
  also sees through an active toggle so shroud checks agree with the redraw.

Behavior classification

Both are fixed: they correct behavior that conflicts with the project's
goals (a crash, and a non-functional debug control). No compatibility
boundary is affected — this is runtime logic only, with no change to saves,
replays, network protocol, formats, or configuration.

Documentation

Added two manual change records under manual/changes/, both targeting
release 0.2.0:

• stack-overflow-on-mass-shroud-unshroud.md (system: map-visibility)
• f4-debug-unshroud-toggle.md (command: fixed:debug-unshroud)

Validation

• Verified in-game that mass shroud/unshroud doesn't cause a StackOverflowException anymore and the F4 toggle when using PLAYTEST command line parameter works in singleplayer.