build: split gix/registry-file features, drop gix facade dependency

Per ADR-012 §4: the gix feature now gates only the engine component
crates (odb/pack/ref/object/fsck); the gix facade crate (client
entry point) is removed — the impls drive components directly
(POC-2's shape). registry-file added as an independent default-on
seam (file-backed registry + record ops, no gitoxide). Both
default-on preserves the batteries-included story; --no-default-
features remains the wire-only embed.

Verification: cargo test (default + --no-default-features), clippy
--all-targets -D warnings, fmt --check.

Cargo.toml

@@ -14,17 +14,22 @@ name = "alkgit"
 path = "src/lib.rs"
 
 [features]
-default = ["gix"]
-# gitoxide-backed backend implementations (registry, refs, pack gen/ingest).
-# Disable for a wire/protocol-only embed with your own backends.
+default = ["gix", "registry-file"]
+# gitoxide-backed engine implementations (refs, pack gen/ingest). The
+# `gix` facade crate is NOT a dependency (ADR-012 §4) — the impls drive
+# the component crates directly (POC-2's shape). Disable for a
+# wire/protocol-only embed or with your own object-storage backend.
 gix = [
-    "dep:gix",
     "dep:gix-odb",
     "dep:gix-pack",
     "dep:gix-ref",
     "dep:gix-object",
     "dep:gix-fsck",
 ]
+# File-backed GitRegistry/GitRegistryStore default (per-repo record
+# files + in-memory index) + the `git/repo/*` op set (ADR-012 §2–3).
+# No gitoxide. Disable to bring your own registry backend.
+registry-file = []
 # sha256 passthrough (untested end-to-end; sha1 is pinned by default — OQ-05).
 # gix-hash stays always-on with sha1: the wire layer needs it, and gix-hash
 # fails to compile with neither hash feature selected (compile-time-rejected
@@ -33,7 +38,6 @@ sha256 = ["gix-hash/sha256"]
 
 [dependencies]
 alkcall = "0.8"
-gix = { version = "0.87", optional = true, default-features = false, features = ["sha1"] }
 gix-odb = { version = "0.84", optional = true, default-features = false, features = ["sha1"] }
 gix-pack = { version = "0.74", optional = true, features = ["sha1"] }
 gix-ref = { version = "0.67", optional = true }