Docs: script stack frames and $.fileName name the file of the code

docs/mcp.md's line and stack row said $.stack shows at most 64 lines
and uses [global] only for the script's own code. It keeps the
innermost 64 frames after a leading line "…" when some are left out,
and [global] names any code outside a function, including code that
an included file, $.evalFile or eval runs. The row now also says how
a frame's file compares with $.fileName in each case: the same file
for code from a file, the run's name where $.fileName is empty for a
script given as source, [eval] where $.fileName is the file being run,
and [unknown] at line 0 where $.fileName is empty.

The engineName row now gives the #targetengine name, and the
script.run params text, the script module doc and the Scripts evidence
line say $.fileName is the file of the code that reads it rather than
the file being run.

crates/engine/src/script/mod.rs

@@ -26,7 +26,7 @@
 //! Before a run starts, its preprocessor lines are read ([`include`]): included files take the
 //! place of their lines, and an error in one is reported at its own file, line and column.
 //! `$.evalFile(path)` runs another file in the same global environment and gives its last value,
-//! `$.fileName` is the file being run and `$.includePath` the `#includepath` list.
+//! `$.fileName` is the file of the code that reads it and `$.includePath` the `#includepath` list.
 //!
 //! A run writes to the Script Console with `$.write` and `$.writeln`, and `vc.log` adds a line
 //! there too. The session's console gets a start line, the run's output, its uncaught error, an end