Clear the -Wall -Wextra warning backlog

[HugoFara]

#156 turns on -Wall -Wextra for ForeFire's own sources, informational rather than fatal, with -DFOREFIRE_WARNINGS_AS_ERRORS=ON for whoever is clearing a file. This issue tracks emptying the backlog.

Linux/GCC: 428 warning lines, 373 unique after deduplicating header warnings repeated across translation units.

Category	Count
-Wunused-parameter	314
-Wdeprecated-copy	65
-Wmissing-field-initializers	24
-Wreorder	9 lines / 3 sites
-Wunused-variable	7

The 65 -Wdeprecated-copy are one missing operator. All trace to FFPoint, which declares a copy constructor (FFPoint.h:42) and a virtual destructor (line 39) but no copy assignment. Its members are three doubles with no owned memory, so the implicit assignment is behaviourally correct — a Rule of Three violation, not a live bug. Declaring FFPoint& operator=(const FFPoint&) clears 15% of the backlog in one line.

The 3 -Wreorder sites are benign (BurningRatioLayer.h:49, RosLayer.h:50, one more): every member is initialised from a constructor parameter, not from another member. Worth fixing anyway so the warning stays able to catch the dangerous version.

Where they concentrate:

46  src/BurningRatioLayer.h      25  src/NCXYZTDataLayer.h
32  src/Command.cpp              24  src/stb_image_write.h
27  src/FluxLayer.h              23  src/RosLayer.h
25  src/TimeGradientDataLayer.h  23  src/GradientDataLayer.h
25  src/PropagativeLayer.h       23  src/FireDomain.cpp

The DataLayer subclasses dominate and share a shape: the unused parameters are interface methods a given layer type genuinely does not need. [[maybe_unused]] or an omitted parameter name, repeated — good first-contribution work.

Suggested order

1. Move vendored src/stb_image_write.h (1724 lines, 24 warnings) and generated src/colormap.h (2883 lines) into third_party/, which Unit tests for every propagation and flux model, and -Wall -Wextra #156 creates. Both are currently globbed in by file(GLOB_RECURSE SRC_FILES src/*.cpp).
2. The FFPoint one-liner.
3. The layer headers.
4. Command.cpp and FireDomain.cpp.

Turn on FOREFIRE_WARNINGS_AS_ERRORS per file as each is cleared, repository-wide at zero. Worth finishing before the C++17 move so the two diffs do not mix.

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Drafted by Claude Opus 5 from a codebase audit. Reviewed by a maintainer before filing.

EDIT: rewrote for human readability.