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/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: []