fix(cj-scan): suppress detection inside nested non-cj begin_* blocks

cj-scan.py matched =#+begin_src cj:= / =#+end_src= line-by-line without awareness of enclosing block scopes. A cj fence embedded inside =#+begin_example= (typical when documenting what the <cj yasnippet emits) or =#+begin_src snippet= (the yasnippet definition itself) was misclassified as a live cj annotation. Two false positives surfaced from a /respond-to-cj-comments run against an org file with yasnippet docs.

Track an active wrapper_type. When the scanner sees =#+begin_<type>= for any type other than cj: (the cj-open regex is checked first), enter a wrapper state where every line is content until the matching =#+end_<type>= closer fires. Inside a wrapper, both fence patterns and legacy inline cj: lines stay suppressed. Added the TestCjScanNestedFencesIgnored class with 6 tests: nesting inside example, src <other-lang>, and quote; regression guards for clean wrapper close and unclosed-wrapper non-swallow. Canonical pytest: 302 passed, 1 skipped.

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