Tui::Source kept the script position in single member fields
(loading_lines/idx/...), so a sourced line that was itself `source inner`
overwrote them: when the inner file finished, the outer script's remaining
lines were silently dropped.
The state is now a stack of SourceFrames — the stack is the call chain. A
nested source just pushes a frame (the running driver, ticker thread or
headless drain, picks it up next line) and the caller's frame resumes when
it pops. Each frame still prints its own "source: <file> (N line(s))"
summary; an interactive-line abort clears the whole chain; depth capped at
32 like the core script engine. The Computing modal shows the top frame.
Regression-tested headless via BootDispatch (tests/tui/test_source.cpp):
nested-then-continue, and self-recursion hitting the depth guard.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
