Add git sources, check-updates, repos submodule, improve docs

dpack features:
- Git source support: packages can specify [source].git for cloning
  instead of tarball download. Supports branch, tag, and commit pinning.
  SHA256 can be set to "SKIP" for git sources.
- check-updates command: queries upstream APIs (GitHub releases/tags)
  to find available updates. Packages set [source].update_check URL.
- CheckUpdates CLI subcommand wired into main.rs

Package changes:
- FreeCAD updated to weekly-2026.03.19 development builds
- dwl: added update_check URL and git source documentation
- src/repos extracted to standalone git repo (danny8632/repos.git)
  and added as git submodule

Documentation:
- All 7 README.md files updated with detailed requirements sections
  including which Linux distros are supported, exact package names
  for Arch/Ubuntu/Fedora, and clear notes about which components
  require Linux vs can be built on macOS
- dpack README: added git source and check-updates documentation

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

README.md

@@ -82,10 +82,38 @@ The project is built in sequential phases. Each phase has clear deliverables and
 
 ### Prerequisites
 
-- A Linux host system for cross-compilation (or the target machine itself)
-- GCC 15.2.0+ (for `-march=znver5` support)
-- Rust toolchain (for building dpack)
-- ~50GB disk space for the build
+Building DarkForge requires a Linux host system. The toolchain bootstrap (Phase 0) can be done from any modern Linux distro, but later phases require the DarkForge chroot/live environment.
+
+**Host system requirements (for building the toolchain):**
+
+Any reasonably modern Linux distribution works. Tested on:
+- Arch Linux (recommended — rolling release with fresh packages)
+- Ubuntu 22.04+ / Debian 12+
+- Fedora 38+
+
+Required host packages:
+
+```bash
+# Arch Linux
+sudo pacman -S base-devel git wget python rust
+
+# Ubuntu / Debian
+sudo apt install build-essential git wget python3 curl bison gawk m4 texinfo
+
+# Fedora
+sudo dnf groupinstall "Development Tools" && sudo dnf install git wget python3 rust cargo
+```
+
+**Tool versions:**
+- GCC 14.1+ or GCC 15.x (for `-march=znver5` support)
+- Rust 1.75+ (for building dpack — install via https://rustup.rs)
+- Python 3.10+
+- GNU Make 4.x+
+- ~50GB free disk space for the full build
+- ~4GB RAM minimum (16GB+ recommended for parallel builds)
+
+**For kernel building (Phase 4) and later phases:**
+These run inside the DarkForge chroot or on the target machine itself. The toolchain built in Phase 0 provides everything needed.
 
 ### 1. Build dpack