fix(core): accept structured extension source in registry

[PhanCongVuDuc]

Problem

With spec-kit 1.0.12, spectatui fails to start in any project where an extension was installed or updated from a catalog:

Error: failed to discover project

Caused by:
    0: failed to parse extensions registry
    1: invalid type: map, expected a string at line 26 column 16

spec-kit now writes source in .specify/extensions/.registry as an object when the extension comes from a catalog (src/specify_cli/extensions/__init__.py, ExtensionManager registry update):

"assess": {
  "version": "1.0.1",
  "source": { "kind": "catalog", "catalog": "default" },
  ...
}

Entries installed locally still use the legacy string ("source": "local"), so a single registry can contain both shapes. ExtensionRegistryEntry.source was Option<String>, so the object makes the whole registry fail to parse and discovery aborts.

Reproduce: specify extension update (or specify extension add <id> from the default catalog) with spec-kit 1.0.12, then run spectatui.

Fix

• ExtensionRegistryEntry.source is read as Option<serde_json::Value>.
• A new extension_source() maps both shapes to ExtensionSource:
  • legacy string: unchanged behaviour ("local" → Local, http… → Url, anything else → Catalog)
  • object: {"kind": "catalog", "catalog": "<name>"} → Catalog(name); {"kind": "local"}, a blank catalog name or an unknown kind → Local — the same normalisation spec-kit applies in _installed_list_json._normalized_source
• An unrecognised shape now falls back to Local instead of failing project discovery.

The presets registry has the same new object source, but PresetRegistryEntry does not declare the field, so serde already ignores it there — no change needed.

Tests

• loads_extensions_with_legacy_and_structured_sources — writes a registry containing one legacy and one structured entry and runs load_extensions. Confirmed it fails on develop with the exact error above, and passes with the fix.
• maps_every_extension_source_shape — covers each string and object variant, plus None.

Checked locally (Windows, stable-x86_64-pc-windows-gnu):

• cargo fmt --all -- --check
• cargo clippy --workspace -- -D warnings
• cargo test --workspace (76 passed)
• release build started against a real spec-kit 1.0.12 project: the v1.1.0 binary exits with the error above, the patched one starts normally.

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