Add learned size limits and Fit remaining space

[kiteretsu903]

I added learned window size limits and an option to fit a new snap beside an existing window:

• Verified size limits are kept as hints for later snaps and previews. They can be inspected, cleared, or saved across restarts in Window Size Limits.
• Fit remaining space fills the space beside an already-snapped window without resizing that window. This option is off by default.
• Reaching a minimum size still allows placement. I also shortened the warning and updated its appearance; it reuses the existing warning switch.

Depends on #1886. The diff currently includes prerequisite changes.

[kiteretsu903]

Closing in favor of the focused replacements: #1902, #1903.

The changes have been reorganized to keep each feature easier to review and limit changes to shared infrastructure. Prerequisite-dependent replacements are drafts and explain which earlier changes are included. The original discussion remains here for reference.