Each executable source block now shows a "Run" button in the outline.
Clicking it runs the block server-side and renders the inserted
#+RESULTS: in place, with an exit-status indicator.
DocumentModel gains executeBlock(row, blockIndex), which runs
/ctl execute <doc> <id> <n>, refreshes the section body, and emits
dataChanged(BodyRole). isLanguageSupported(lang) backs button visibility
via /ctl languages (loaded once, cached).
OutlineView tags each parsed code part with its src-block ordinal so the
GUI index matches the server's block indexing, and the code block header
carries the Run control plus running/ok/error status.
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