feat(core): QUIT

The CORE word was missing. QUIT empties the return stack, enters
interpretation state, restores SOURCE-ID to the user input device and
returns to the interpreter without a message, leaving the data stack
untouched -- that last part is the whole difference to ABORT, which the
standard defines as 'empty the data stack, then QUIT'.

Implemented on the throw plumbing with the standard code -56, so nested
EVALUATE / INCLUDE frames unwind and are abandoned on the way out. Two
places treat -56 specially: CATCH lets it through (QUIT is a return to
the prompt, not an exception) and evaluate() turns it into a silent Ok
after the compile-state wipe it already performs.

Semantics checked against gforth 0.7.3 and SwiftForth sf64, which agree:
the data stack survives, nothing is printed, the rest of the input is
abandoned, and '1 2 ' QUIT CATCH .' prints nothing while leaving 1 2.
Six tests in outer.rs pin it. Deliberately NOT added to the cross-engine
corpus: what QUIT abandons is the input source, and the three engines are
fed differently there, so a comparison would measure the harness.

The gap survived because the Forth 2012 suite skips QUIT by its own
admission, and HELP's coverage lint compares dictionary against docs --
a word missing from both looks complete. docs/wafer-anki.txt had been
documenting QUIT as if it existed.

ABORT itself was already correct: executed while a definition is open it
clears both stacks and returns to interpretation state.

Ships as v0.2.5.

crates/core/src/wordhelp.rs

@@ -716,6 +716,11 @@ pub const WORD_DOCS: &[(&str, &str, &str)] = &[
         "Read a line of input (unsupported here).",
     ),
     ("ABORT", "( i*x -- )", "Empty the stacks and abort."),
+    (
+        "QUIT",
+        "( -- ) ( R: i*x -- )",
+        "Empty the return stack, return to the interpreter; data stack kept.",
+    ),
     (
         "ABORT\"",
         "( flag -- )",