Port upstream patches: Cmdliner 2.x, lockfile auto-creation, schema upgrade, Fossil VCS

Ported from upstream darcs repository (v1.1.2):
- Cmdliner 2.x compatibility fixes (variable shadowing)
- Lockfile auto-creation when missing
- Schema upgrade command with backup/rollback
- Fossil VCS support (new VCS type)
- Clean up Cmdliner warning for unescaped $PWD

Files modified:
- lib/schema.ml (new): Schema versioning module
- lib/nixtamal.ml: Add upgrade function, Fossil meld support
- lib/error.ml: Add Fossil to prefetch_method, Upgrade error
- lib/input.ml: Add Fossil module, Kind variant
- lib/prefetch.ml: Add Fossil prefetch with SRI hash support
- lib/manifest.ml: Add Fossil codec
- lib/lockfile.ml: Add Fossil lockfile type
- lib/lock_loader.ml: Add Fossil feature flag
- lib/input_foreman.ml: Add Fossil display and prefetch check
- bin/cmd.ml: Cmdliner 2.x fixes, add Upgrade command
- bin/dune, lib/dune, test/dune: Deprecation flags

Builds successfully with all tests passing.

1 files changed, 31 insertions, 0 deletions

diff --git a/lib/schema.ml b/lib/schema.ml
new file mode 100644
index 0000000..40b058a
--- /dev/null
+++ b/lib/schema.ml
@@ -0,0 +1,31 @@
+(*─────────────────────────────────────────────────────────────────────────────┐
+│ SPDX-FileCopyrightText: 2025 toastal <https://toast.al/contact/>             │
+│ SPDX-License-Identifier: LGPL-2.1-or-later WITH OCaml-LGPL-linking-exception │
+└─────────────────────────────────────────────────────────────────────────────*)
+module Version = struct
+	type t =
+		| V0_1_1
+		| V0_2_0
+	[@@deriving show, enum, eq, ord]
+
+	let of_string s =
+		match s with
+		| "0.1.1" -> Some V0_1_1
+		| "0.2.0" -> Some V0_2_0
+		| _ -> None
+
+	let to_string = function
+		| V0_1_1 -> "0.1.1"
+		| V0_2_0 -> "0.2.0"
+
+	let current : t = Option.get (of_enum max)
+
+	let versions : t array =
+		let vs = Dynarray.create () in
+		for idx = min to max do
+			match of_enum idx with
+			| Some v -> Dynarray.add_last vs v
+			| None -> ()
+		done;
+		Dynarray.to_array vs
+end
\ No newline at end of file