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)

meson.build

@@ -0,0 +1,127 @@
+project(
+  'depot',
+  version: '0.1.0',
+  meson_version: '>=0.60.0',
+)
+
+fs = import('fs')
+
+cargo = find_program('cargo', required: true)
+find_prog = find_program('find', required: true)
+sh = find_program('sh', required: true)
+
+src_root = meson.project_source_root()
+build_root = meson.project_build_root()
+src_root_prefix = src_root + '/'
+
+cargo_profile = 'debug'
+cargo_release = false
+if get_option('buildtype') != 'debug'
+  cargo_profile = 'release'
+  cargo_release = true
+endif
+
+cargo_inputs = files('Cargo.toml')
+if fs.is_file('Cargo.lock')
+  cargo_inputs += files('Cargo.lock')
+endif
+
+cargo_src_list = run_command(
+  find_prog,
+  join_paths(src_root, 'src'),
+  '-type',
+  'f',
+  '(',
+  '-name',
+  '*.rs',
+  '-o',
+  '-name',
+  '*.toml',
+  ')',
+  check: true,
+  capture: true,
+).stdout().strip()
+foreach rel : cargo_src_list.split('\n')
+  if rel == ''
+    continue
+  endif
+  if rel.startswith(src_root_prefix)
+    rel = rel.replace(src_root_prefix, '')
+  endif
+  cargo_inputs += files(rel)
+endforeach
+
+depot_bin = custom_target(
+  'depot-bin',
+  input: cargo_inputs,
+  output: 'depot',
+  command: [
+    sh,
+    join_paths(src_root, 'tools', 'meson', 'cargo-build.sh'),
+    cargo,
+    src_root,
+    build_root,
+    cargo_profile,
+    cargo_release ? '1' : '0',
+    '@OUTPUT@',
+  ],
+  console: true,
+  build_by_default: true,
+  install: true,
+  install_dir: get_option('bindir'),
+)
+
+test(
+  'cargo-test',
+  sh,
+  args: [
+    join_paths(src_root, 'tools', 'meson', 'cargo-test.sh'),
+    cargo.full_path(),
+    src_root,
+    build_root,
+  ],
+  suite: ['cargo'],
+)
+
+# System configuration examples installed under the actual runtime lookup paths.
+install_data(
+  'contrib/depot.toml',
+  install_dir: get_option('sysconfdir'),
+)
+
+example_sysconf_dir = join_paths(get_option('sysconfdir'), 'depot.d')
+example_depotd_list = run_command(
+  find_prog,
+  join_paths(src_root, 'contrib', 'depot.d'),
+  '-maxdepth',
+  '1',
+  '-type',
+  'f',
+  '-name',
+  '*.toml',
+  check: true,
+  capture: true,
+).stdout().strip()
+example_depotd_files = []
+foreach rel : example_depotd_list.split('\n')
+  if rel == ''
+    continue
+  endif
+  if rel.startswith(src_root_prefix)
+    rel = rel.replace(src_root_prefix, '')
+  endif
+  example_depotd_files += files(rel)
+endforeach
+if example_depotd_files.length() > 0
+  install_data(example_depotd_files, install_dir: example_sysconf_dir)
+endif
+
+# Per-user example is documentation, not a system config file.
+install_data(
+  'contrib/user.depot.toml.example',
+  install_dir: join_paths(get_option('datadir'), 'doc', meson.project_name(), 'examples'),
+)
+install_data(
+  'contrib/README.md',
+  install_dir: join_paths(get_option('datadir'), 'doc', meson.project_name()),
+)