File the scanned line in RUNSTREAM, not the raw one

process_line_runstream() chose between the stripped line and the raw buffer
on obj->mode == AKBASIC_MODE_REPL, but nothing reaches that function except
the RUNSTREAM arm of step(). The stripped branch was dead, so every program
the driver ran from a file was stored with its own line number inside
source[]: LIST printed "10 10 PRINT", and DSAVE and HELP would have echoed
it the same way.

No .bas in either corpus calls LIST, and runtime_verbs.c's load() helper --
which claims to load "the way RUNSTREAM would" -- already stored the scanned
line, so it encoded the correct contract and could not catch the wrong one.
The new test drives the real sink path instead.

Keep the step-over-a-leading-number allowance in scan_line_labels() and
skip_lineno(): no path inside the library stores a raw line now, but
akbasic_runtime_store_line() is public and a host may hand it one.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/structtype.c

@@ -42,9 +42,9 @@ static const char *skip_space(const char *cursor)
 /**
  * @brief Step over a stored line's own line number, if it still carries one.
  *
- * A line arrives either from akbasic_runtime_load(), which files what the
- * scanner already stripped, or from RUNSTREAM, which files the raw text. Both
- * spellings are real, so both are handled here -- the same allowance
+ * Every path inside the library files what the scanner already stripped, but
+ * akbasic_runtime_store_line() is public and a host may hand it a raw line, so
+ * both spellings are real and both are handled here -- the same allowance
  * scan_line_labels() makes.
  */
 static const char *skip_lineno(const char *cursor)