feat: add repo planning/binary repo support and Meson build integration

- add repos.toml source/binary repo configuration with arch overrides and priorities
- add dependency planner/execution plan flow with interactive provider selection and --yes support
- expand binary repo DB metadata (sha512, dependencies, file lists) and repo owns/search helpers
- add rootfs-scoped fd locking and minisign detached signing/verification helpers
- add Meson build/test support for the project with builddir-local CARGO_HOME/CARGO_TARGET_DIR
- install contrib example configs into sysconfdir (/etc/depot.d layout) via Meson
- improve builder env handling (sanitized PATH, tool env passthrough, CXXFLAGS support)
- support += appends in package specs and add build.flags.cxxflags parsing
- prompt for git credentials when private repo auth is required
- update docs/examples and dependency setup (system libs, zstdmt, tempfile pin)

contrib/depot.d/repos.toml

@@ -0,0 +1,26 @@
+[settings]
+prefer_binary = true
+
+[source.vertex]
+url = "https://gitlab.com/vertex-linux/packages.git"
+enabled = true
+priority = 50
+subdirs = ["core", "extra"]
+
+[binary.core]
+url = "https://repo.sfgos.net/core"
+enabled = true
+priority = 10
+repo_db = "repo.db.zst"
+
+[binary.core.arch.x86_64]
+enabled = true
+
+[binary.extra]
+url = "https://repo.sfgos.net/extra"
+enabled = true
+priority = 20
+repo_db = "repo.db.zst"
+
+[binary.extra.arch.x86_64]
+enabled = true