[Build] enable IPO/LTO for release builds

[Harmenszoon]

Summary

• add SC_ENABLE_LTO CMake option, default ON
• use check_ipo_supported() and skip gracefully when IPO/LTO is unavailable
• enable target-level IPO/LTO for non-Debug builds on engine, simc, and SimulationCraft

Why

CMake Release builds currently use the platform defaults only. On MSVC + Ninja this produced /O2 /Ob2 /DNDEBUG without /GL or /LTCG. A no-code A/B showed that enabling CMake IPO/LTO produces output-equivalent sims with a consistent runtime win.

Validation

Toolchain: Windows, MSVC 19.44.35217.0, CMake 3.31.6, Ninja, BUILD_GUI=OFF.

Initial no-code A/B, same source, baseline Release vs -DCMAKE_INTERPROCEDURAL_OPTIMIZATION=ON, threads=1 target_error=0 deterministic=1 seed=123456 cleanup_threads=1, 5 interleaved repeats. DPS and TotalEvents matched exactly in all valid cases.

Profile	Mean win	Median win
profiles/CI.simc iterations=10	5.48%	6.23%
Windwalker DungeonSlice 2000	9.26%	10.65%
Windwalker ST 2000	12.48%	12.21%
Shadow Priest ST 2000	10.40%	10.35%
Havoc DH 5T 2000	8.91%	9.47%
Dev Evoker 5T 2000	9.05%	10.32%

The requested full profiles/CI.simc fight_style=DungeonSlice control is invalid on this checkout because Demon Hunter disables DungeonSlice by default (engine/class_modules/sc_demon_hunter.cpp:11273), and the hinted enable_dungeon_slice=1 is a Demon Hunter actor option rather than a global profile option. I used Windwalker DungeonSlice as the substituted DungeonSlice control.

Final validation on this branch, based on current origin/midnight (cfbfbe4):

• cmake -S . -B build-pr-lto-off -G Ninja -DCMAKE_BUILD_TYPE=Release -DBUILD_GUI=OFF -DSC_ENABLE_LTO=OFF
• cmake --build build-pr-lto-off --parallel
• cmake -S . -B build-pr-lto-on -G Ninja -DCMAKE_BUILD_TYPE=Release -DBUILD_GUI=OFF
• cmake --build build-pr-lto-on --parallel

Confirmed SC_ENABLE_LTO=OFF has no /GL or /LTCG; default ON has /GL compile flags and /LTCG link flags. The existing MSVC warning at engine/class_modules/sc_druid.cpp:8909 appears unchanged in both builds.

Final A/B, 5 interleaved repeats:

Profile	DPS / TotalEvents	Mean win	Median win
profiles/CI.simc iterations=10	exact match	5.50%	4.65%
Windwalker ST 2000	exact match	10.66%	10.95%

Risk

• Release build time increases on my machine: earlier full build comparison was ~36.9s baseline vs ~59.8s IPO.
• Some packagers may prefer faster local builds or smaller toolchain requirements, so this is opt-out with -DSC_ENABLE_LTO=OFF.
• Unsupported IPO/LTO toolchains print a status message and continue without enabling IPO/LTO.
• GUI target has the same target-level property applied, but my local validation was CLI-only with BUILD_GUI=OFF.