feat!: optional fail-closed fallback, legend output, richer prompt & docs

BREAKING CHANGE: regex fallback is now opt-in (patterns no longer defaults
to presets.swiss). With no fallback an LLM failure throws AnonymizationError
(fail-closed) and the pre-filter is bypassed; at least one of llm/patterns is
required. AnonymizationResult gains a required `legend`; anonymizeChunks seed
is now { mapping, legend? } and returns legend.

- prompt: model may coin new UPPERCASE abbreviations and returns a 'legende'
  explaining every abbreviation used (French); backfilled by DEFAULT_LEGEND
- PatternDef.meaning surfaces in the legend; swiss/generic presets get meanings
- AnonymizationError (exported) wraps the cause on fail-closed
- README: drop the chatbot provenance line; add 'How it works' + nLPD sections
- 34 tests / 99% coverage; bump to 0.2.0

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tsconfig.json

@@ -1,9 +1,9 @@
 {
   "compilerOptions": {
-    "target": "ES2021",
+    "target": "ES2022",
     "module": "ESNext",
     "moduleResolution": "Bundler",
-    "lib": ["ES2021", "DOM"],
+    "lib": ["ES2022", "DOM"],
     "declaration": true,
     "strict": true,
     "noUnusedLocals": true,