new: template manifests and registry infrastructure

Split the Template trait into a data half and a logic half. Every
template is now declared by a manifest under data/templates/<id>/
(CLI id, wizard label, detection markers, Build-Depends, architecture,
rules dh line, rules-extra body, control source fields, gitignore
entries and static file bodies with {placeholder} substitution),
embedded through the TEMPLATE_SOURCES index and parsed once into the
registry; the order of the index is the wizard menu order and the
detection priority at once. The logic half is the slim TemplateHooks
trait (probe, post_write, file-body overrides merged over the manifest
bodies by path shadowing, Build-Depends/architecture amendments and
extra context values), registered per template as a HOOKS static: a
template without hooks needs zero Rust.

- TemplateId becomes a Copy wrapper of the stable CLI string; the
  enum, its all/as_str/display_name/from_label matches and the old
  statics array collapse into the registry accessors.
- rust's rules overrides move to data/templates/rust/rules.extra.tpl
  with {locked}/{artifact} hook context; python's backend table,
  meson/cmake's pkg-config opt-in, autotools' gettext and python's
  C-extension hints become hook amendments over the manifest baseline.
- detect.rs drops its hardcoded marker cascade: the manifests'
  detect.files drive detection in registry order, with the shell
  single-script heuristic and the never-detected empty template kept
  as the code special cases they are. License sniffing is untouched.
- The template tests port to manifest validation: registry coverage
  and stable order, placeholder presence in the rendering context,
  rules composition, the Build-Depends/architecture/dh-line table now
  asserted against the manifest data, and the hook shadowing merge.

The static skeleton bodies of the shell/empty/makefile/go templates
stay in their Rust hooks for now; the next commit moves them into
their manifests.

data/templates/autotools/manifest.yml

@@ -0,0 +1,19 @@
+## The `autotools` template: a C project with a configure.ac built through
+## debhelper's auto-detection (dh runs autoreconf itself when it finds
+## configure.ac, debhelper >= 10 — no override needed). The logic half —
+## the AC_INIT probe, the skeleton bodies and the gettext detection
+## (appended to Build-Depends) — lives in src/new/templates/autotools.rs.
+##
+## Schema: see src/new/templates/mod.rs.
+
+id: autotools
+label: C/C++ (Autotools)
+detect:
+  files: [configure.ac]
+build_depends:
+  - autoconf
+  - automake
+  - libtool
+architecture: any
+rules_dh_line: "dh $@"
+files: []

data/templates/cmake/manifest.yml

@@ -0,0 +1,16 @@
+## The `cmake` template: a C/C++ project built with CMake through the
+## debhelper cmake buildsystem. The logic half — the project() probe, the
+## skeleton bodies and the wizard's pkg-config opt-in (appended to
+## Build-Depends) — lives in src/new/templates/cmake.rs.
+##
+## Schema: see src/new/templates/mod.rs.
+
+id: cmake
+label: C/C++ (CMake)
+detect:
+  files: [CMakeLists.txt]
+build_depends:
+  - cmake
+architecture: any
+rules_dh_line: "dh $@ --buildsystem=cmake"
+files: []

data/templates/empty/manifest.yml

@@ -0,0 +1,14 @@
+## The `empty` template: a metapackage (non-empty Depends list) or an
+## empty base package with no build system at all — pure `dh $@` plumbing
+## as a starting point for hand-written rules.
+##
+## Schema: see src/new/templates/mod.rs.
+
+id: empty
+label: Metapackage / empty base (no build system)
+detect:
+  files: []
+build_depends: []
+architecture: all
+rules_dh_line: "dh $@"
+files: []

data/templates/go/manifest.yml

@@ -0,0 +1,18 @@
+## The `go` template: a Go module built through dh-golang. The logic half
+## — the go.mod module-line probe and the `{go_import_path}` value below —
+## lives in src/new/templates/go.rs.
+##
+## Schema: see src/new/templates/mod.rs.
+
+id: go
+label: Go module
+detect:
+  files: [go.mod]
+build_depends:
+  - golang-any
+  - dh-golang
+architecture: any
+rules_dh_line: "dh $@ --buildsystem=golang"
+source_fields:
+  XS-Go-Import-Path: "{go_import_path}"
+files: []

data/templates/makefile/manifest.yml

@@ -0,0 +1,17 @@
+## The `makefile` template: a generic project driven by a plain Makefile.
+## debhelper's makefile buildsystem runs `make` for the build and
+## `make install DESTDIR=...` when the Makefile carries an `install:`
+## target (missing targets are skipped gracefully), so plain `dh $@`
+## plumbing is enough here.
+##
+## Schema: see src/new/templates/mod.rs.
+
+id: makefile
+label: Generic (Makefile)
+detect:
+  files: [Makefile]
+build_depends:
+  - build-essential
+architecture: any
+rules_dh_line: "dh $@"
+files: []

data/templates/meson/manifest.yml

@@ -0,0 +1,16 @@
+## The `meson` template: a C/C++ project built with Meson through the
+## debhelper meson buildsystem. The logic half — the project() probe, the
+## skeleton bodies and the wizard's pkg-config opt-in (appended to
+## Build-Depends) — lives in src/new/templates/meson.rs.
+##
+## Schema: see src/new/templates/mod.rs.
+
+id: meson
+label: C/C++ (Meson)
+detect:
+  files: [meson.build]
+build_depends:
+  - meson
+architecture: any
+rules_dh_line: "dh $@ --buildsystem=meson"
+files: []

data/templates/python/manifest.yml

@@ -0,0 +1,21 @@
+## The `python` template: a PEP 517 project built with pybuild. The logic
+## half — the pyproject.toml/setup.py probe, the skeleton bodies and the
+## Build-Depends/architecture resolution for existing projects (backend
+## package, pyproject presence, C-extension hints) — lives in
+## src/new/templates/python.rs; the lists below are the fresh-skeleton
+## baseline it starts from.
+##
+## Schema: see src/new/templates/mod.rs.
+
+id: python
+label: Python (pyproject.toml / setup.py)
+detect:
+  files: [pyproject.toml, setup.py, setup.cfg]
+build_depends:
+  - dh-python
+  - python3-all
+  - pybuild-plugin-pyproject
+  - python3-setuptools
+architecture: all
+rules_dh_line: "dh $@ --with python3 --buildsystem=pybuild"
+files: []

data/templates/rust/manifest.yml

@@ -0,0 +1,26 @@
+## The `rust` template: a vendored Cargo build (see the module docs of
+## src/new/templates/rust.rs for the vendoring strategy). The logic half —
+## the cargo vendor post-write hook, the Cargo.toml/src skeletons with
+## their crate-name sanitizing, and the `{locked}` / `{artifact}` values of
+## rules.extra.tpl — lives in that module.
+##
+## Schema: see src/new/templates/mod.rs.
+
+id: rust
+label: Rust (Cargo.toml)
+detect:
+  files: [Cargo.toml]
+build_depends:
+  - cargo:native
+  - rustc:native
+architecture: any
+rules_dh_line: "dh $@"
+# The vendored-build overrides appended to debian/rules; `--locked` is only
+# used when the packaged tree already carries a Cargo.lock (the vendoring
+# hook patches it in once it creates the lockfile), and the built artifact
+# of a fresh skeleton is named after its crate.
+rules_extra_file: rules.extra.tpl
+gitignore_entries:
+  - vendor/
+  - .cargo/config.toml
+files: []

data/templates/rust/rules.extra.tpl

@@ -0,0 +1,19 @@
+override_dh_auto_build:
+	cargo build --release --offline{locked}
+
+override_dh_auto_install:
+	install -Dm755 target/release/{artifact} debian/{name}/usr/bin/{command}
+
+override_dh_auto_test:
+	cargo test --release --offline{locked}
+
+override_dh_update_autotools_config:
+
+override_dh_clean:
+	# dh_clean unlinks `*.orig` patch backups, but vendored crates
+	# ship files like `Cargo.toml.orig` that cargo's per-file
+	# checksums require on cold builds (chroots, Launchpad).
+	dh_clean -X .orig
+
+override_dh_auto_clean:
+	cargo clean

data/templates/shell/manifest.yml

@@ -0,0 +1,16 @@
+## The `shell` template: a single interpreted script installed to
+## /usr/bin with plain `dh $@` plumbing. Detection is not marker-based: the
+## single-script heuristic of src/new/detect.rs (a lone *.sh or shebang
+## file) maps here. The probe pre-filling the wizard answers from the
+## script file name lives in src/new/templates/shell.rs.
+##
+## Schema: see src/new/templates/mod.rs.
+
+id: shell
+label: Shell script / single interpreted file
+detect:
+  files: []
+build_depends: []
+architecture: all
+rules_dh_line: "dh $@"
+files: []