Improve stack-underflow error messages in arithmetic primitives

[DavidLiedle]

Summary

Forth primitives like +, -, *, /, and MOD produce a terse error
when the data stack doesn't have enough operands. For someone who just
typed their first definition wrong, the current message is not very
self-explanatory — they don't know whether they're missing one item or
three, or which word triggered the underflow.

This issue is to improve those messages so they read more like
"+ needs 2 items, stack had 1 (in word SQUARE)" — naming the word and
the shortfall.

Why it's a good first issue

The change is local to src/vm/ (likely the arithmetic primitives module
plus wherever the underflow error is constructed). No cross-module design
decisions, no protocol changes, no new dependencies. It's a great way to
get familiar with how the Forth VM dispatches primitives and how errors
flow back to the REPL.

It's also user-visible: anyone running cargo install unit and typing at
the REPL will benefit on day one.

Acceptance criteria

• At least the binary arithmetic words (+, -, *, /, MOD) and
  the comparison words (=, <, >) report a clearer underflow
  message that names the operand count and, when available, the
  enclosing word.
• The new message text is captured in a unit test (under src/vm/ or
  tests/) so future regressions are caught.
• No change to successful behavior — existing tests still pass.
• WASM REPL output matches native (verify by running the demo).

Where to look

• src/vm/ — interpreter and primitive implementations
• src/integration_tests.rs — pattern for stack/error tests
• Recent fix-pass commits for tone reference: 4696b6f, 92c2e30,
  1855f24