Clear the -Wall -Wextra warning backlog

[HugoFara]

Tracks clearing the -Wall -Wextra backlog enabled in #156. Currently 373 unique GCC warnings:

Warning	Count
-Wunused-parameter	314
-Wdeprecated-copy	65
-Wmissing-field-initializers	24
-Wunused-variable	7
-Wreorder	3 sites (harmless)

• All 65 -Wdeprecated-copy came from FFPoint (fixed in Give FFPoint one way to copy itself #191).
• Most unused parameters are in the DataLayer subclass headers. Fixing them with [[maybe_unused]] or unnamed parameters is good first-contribution work.

Order: move stb_image_write.h to third_party/ (#192), FFPoint (#191), then decide where the generated colormap.h goes, then the layer headers, then Command.cpp / FireDomain.cpp. Enable FOREFIRE_WARNINGS_AS_ERRORS for each file as it is cleared. Finish this before #164.

---

Drafted with Claude Opus 5, reviewed by a maintainer.