chore(setup): build miner from DragonX/drg-xmrig and rename staging dir

setup.sh now clones/builds the miner from git.dragonx.is/DragonX/drg-xmrig
(was dragonx/xmrig-hac), and the prebuilt staging dir is renamed
prebuilt-binaries/xmrig-hac -> prebuilt-binaries/drg-xmrig across build.sh, the
legacy Windows build script, .gitignore and README.

Also aligns setup.sh's section-8 directory list with section 7 (it previously
used a bare "xmrig" token, creating a stray empty prebuilt-binaries/xmrig/),
and updates the xmrig_manager.cpp header comment.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitignore

@@ -11,8 +11,8 @@ prebuilt-binaries/dragonxd-win/*
 !prebuilt-binaries/dragonxd-win/.gitkeep
 prebuilt-binaries/dragonxd-mac/*
 !prebuilt-binaries/dragonxd-mac/.gitkeep
-prebuilt-binaries/xmrig-hac/*
-!prebuilt-binaries/xmrig-hac/.gitkeep
+prebuilt-binaries/drg-xmrig/*
+!prebuilt-binaries/drg-xmrig/.gitkeep
 
 
 # External sources / toolchains (created by scripts/setup.sh)
@@ -33,7 +33,7 @@ imgui.ini
 *.bak*
 *.params
 asmap.dat
-/external/xmrig-hac
+/external/drg-xmrig
 /memory
 /todo.md
 /.github/