feat(auth): brahman-auth — autenticación del escritorio (PAM + mock)

Base del DM/greeter de carmen. Contrato Authenticator agnóstico:
authenticate(usuario, secreto) -> UserInfo (uid/gid/home/shell).
PamAuthenticator verifica contra PAM (/etc/pam.d/carmen); MockAuthenticator
con credenciales en memoria para tests. AuthError grueso: BadCredentials
vs AccountUnavailable, sin filtrar existencia de cuentas. resolve_user
vía getpwnam. data/carmen como servicio PAM; ejemplo auth-probe.

11 tests; el camino PAM real se ejercita.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Cargo.lock

@@ -1522,6 +1522,26 @@ dependencies = [
  "serde",
 ]
 
+[[package]]
+name = "bindgen"
+version = "0.69.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "271383c67ccabffb7381723dea0672a673f292304fcb45c01cc648c7a8d58088"
+dependencies = [
+ "bitflags 2.11.1",
+ "cexpr",
+ "clang-sys",
+ "itertools 0.11.0",
+ "lazy_static",
+ "lazycell",
+ "proc-macro2",
+ "quote",
+ "regex",
+ "rustc-hash 1.1.0",
+ "shlex",
+ "syn 2.0.117",
+]
+
 [[package]]
 name = "bindgen"
 version = "0.71.1"
@@ -1806,6 +1826,16 @@ dependencies = [
  "ulid",
 ]
 
+[[package]]
+name = "brahman-auth"
+version = "0.1.0"
+dependencies = [
+ "nix 0.29.0",
+ "pam",
+ "rpassword",
+ "thiserror 2.0.18",
+]
+
 [[package]]
 name = "brahman-broker"
 version = "0.1.0"
@@ -5086,7 +5116,7 @@ dependencies = [
  "ashpd 0.11.1",
  "async-task",
  "backtrace",
- "bindgen",
+ "bindgen 0.71.1",
  "blade-graphics",
  "blade-macros",
  "blade-util",
@@ -5238,7 +5268,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "05cb8912ae17371725132d2b7eec6797a255accc95d58ee5c1134b529810f14b"
 dependencies = [
  "anyhow",
- "bindgen",
+ "bindgen 0.71.1",
  "core-foundation 0.10.0",
  "core-video",
  "ctor",
@@ -6557,6 +6587,12 @@ dependencies = [
  "spin",
 ]
 
+[[package]]
+name = "lazycell"
+version = "1.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "830d08ce1d1d941e6b30645f1a0eb5643013d835ce3779a5fc208261dbe10f55"
+
 [[package]]
 name = "leak"
 version = "0.1.2"
@@ -9397,6 +9433,40 @@ dependencies = [
  "windows 0.59.0",
 ]
 
+[[package]]
+name = "pam"
+version = "0.8.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8ab553c52103edb295d8f7d6a3b593dc22a30b1fb99643c777a8f36915e285ba"
+dependencies = [
+ "libc",
+ "memchr",
+ "pam-macros",
+ "pam-sys",
+ "users",
+]
+
+[[package]]
+name = "pam-macros"
+version = "0.0.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c94f3b9b97df3c6d4e51a14916639b24e02c7d15d1dba686ce9b1118277cb811"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 1.0.109",
+]
+
+[[package]]
+name = "pam-sys"
+version = "1.0.0-alpha5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ce9484729b3e52c0bacdc5191cb6a6a5f31ef4c09c5e4ab1209d3340ad9e997b"
+dependencies = [
+ "bindgen 0.69.5",
+ "libc",
+]
+
 [[package]]
 name = "parking"
 version = "2.2.1"
@@ -14139,6 +14209,16 @@ version = "2.1.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "daf8dba3b7eb870caf1ddeed7bc9d2a049f3cfdfae7cb521b087cc33ae4c49da"
 
+[[package]]
+name = "users"
+version = "0.10.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "aa4227e95324a443c9fcb06e03d4d85e91aabe9a5a02aa818688b6918b6af486"
+dependencies = [
+ "libc",
+ "log",
+]
+
 [[package]]
 name = "usvg"
 version = "0.45.1"