Initial release: self-hostable APT repository server and CLI

urapt is a self-hostable APT repository server with a companion CLI for
pushing and managing Debian .deb packages.

Server (urapt-server):
- REST API + APT endpoint, SQLite storage (pure-Go modernc driver, no CGO)
- .deb files stored content-addressed on disk, reference-counted for dedup
- Server-managed RSA-4096 OpenPGP signing key (ProtonMail/go-crypto)
- APT indices (Release/InRelease/Packages[.gz/.xz]) generated on demand
  from the DB, cached in memory, signed with the server key
- Full APT model: repositories -> distributions -> components -> architectures
- Bearer-token auth for REST; HTTP Basic auth for private-repo APT reads
- First registrant becomes admin; repo-scoped permissions
  (read/write/read-write/admin) plus owner and server-admin roles
- Multipart package push with control-field extraction, list/show/delete,
  pool serving, blob ref-count cleanup
- Audit log

CLI (urapt):
- register/login/logout/whoami, token management
- repo/distro/component/arch CRUD, member management
- push/pull/ls/show/rm for packages
- apt-config helper that emits apt setup commands (key, sources.list,
  auth.conf for private repos)

Packaging & docs:
- Dockerfile (multi-stage distroless), docker-compose.yml, sample config
- README quick start, architecture overview, config reference, security notes
- PLAN.md design blueprint, CHANGELOG.md, GPL-3.0 LICENSE
- GitHub Actions CI (test, lint, cross-build for linux/darwin amd64/arm64)
- Makefile release target producing static binaries + tarballs + checksums

Tests cover the data-access layer, auth/permission checks, APT index
generation, .deb parsing, GPG signing, the REST API, and the typed API
client. Verified end-to-end on a Raspberry Pi (arm64) pushing and installing
a real package.

shared/apt/pool.go

@@ -0,0 +1,28 @@
+package apt
+
+import "strings"
+
+// PoolPath computes the conventional Debian pool path for a package:
+// pool/<component>/<letter>/<src>/<filename>, where <src> is the source package
+// name (or the binary package name if absent) and <letter> follows the Debian
+// "libX" convention.
+func PoolPath(component, source, packageName, filename string) string {
+	src := source
+	if src == "" {
+		src = packageName
+	}
+	letter := poolLetter(src)
+	return strings.Join([]string{"pool", component, letter, src, filename}, "/")
+}
+
+// poolLetter returns the pool subdirectory prefix for a source name: "lib" +
+// next char for names starting with "lib", otherwise the first character.
+func poolLetter(src string) string {
+	if len(src) >= 4 && strings.HasPrefix(src, "lib") {
+		return "lib" + string(src[3])
+	}
+	if src == "" {
+		return "0"
+	}
+	return string(src[0])
+}