refactor: deep review fixes round 3

- client.py:
  - Remove dead _extract_cursor second branch (unreachable code)
  - Cache SSL context as module-level _SSL_CTX (avoid re-reading CA certs)
  - Add 404 stale-fallback retry to _graphql_post (parity with GET)
  - Remove dead core.get('name')/core.get('screen_name') in fetch_user
  - Set Content-Type: application/json only for POST requests
  - Rename _to_int → _parse_int for clarity vs config._as_int
  - Add 'not thread-safe' note on module-level caches

- cli.py:
  - _fetch_and_display now accepts optional config param (fix double load)
  - Refactor user_posts to use _fetch_and_display
  - Pass config to all _fetch_and_display callers

- pyproject.toml:
  - Move xclienttransaction/requests to optional [transaction] deps
  - Add beautifulsoup4 to [transaction] optional deps

- README.md:
  - Add rateLimit config section with comments
  - Add constants.py to project structure tree

README.md

@@ -140,6 +140,12 @@ filter:
     replies: 2.0
     bookmarks: 5.0
     views_log: 0.5
+
+rateLimit:
+  requestDelay: 1.5     # seconds between paginated requests
+  maxRetries: 3          # retry count on rate limit (429)
+  retryBaseDelay: 5.0    # base delay for exponential backoff
+  maxCount: 200          # hard cap on fetched items
 ```
 
 Filter behavior:
@@ -199,6 +205,7 @@ twitter_cli/
 ├── client.py
 ├── auth.py
 ├── config.py
+├── constants.py
 ├── filter.py
 ├── formatter.py
 ├── serialization.py