Remove leftover build files nothing reads anymore

MANIFEST, Assembly (Detroit's config), .index, Indexfile and
lib/core/facets.yml belonged to older Rubyworks tooling (mast, index,
Detroit). The gemspec builds its own file list and metadata, so nothing
read them, and they had drifted: MANIFEST was missing files and .index
still said version 3.1.1.

The Rakefile only used .index for the docs version; it now uses
Facets::VERSION. The `prepare` task, which just regenerated MANIFEST
and .index with mast and index, is gone, so `rake package` builds the
gem without needing either tool.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E2Njj4j1r9twkVcXsMySLW

Author: claude

.index

@@ -72,6 +72,10 @@ Changes:
 * Internal
 
   * Run the qed demos in CI.
+  * Remove leftover build files that nothing reads anymore: `MANIFEST`,
+    `Assembly` (Detroit), `.index`, `Indexfile` and `lib/core/facets.yml`.
+    The Rakefile takes its version from `Facets::VERSION` (`.index` still said
+    3.1.1), and `rake package` no longer needs the `mast` and `index` tools.
   * Fix the qed demos, which had 8 failures and 2 errors. Most were stale
     expectations (`Array#from`, `#step`, `#arrange`, `String#similarity`)
     and the date demo held pasted irb output.