x/y/z bindings not restored when a function body throws #9

From the Codex whole-repo review on #5 (review comment).

Location: imp-eval.mts:495 on main (6813323)

[P2] Restore parameter bindings when a function body throws

If lastEval throws, execution skips restoration of the saved x, y, and z bindings, leaving function arguments in the shared word dictionary. In separate evaluations, x: 10, f: {x + bad}, and f[99] demonstrate this: after catching the undefined-word error, evaluating x returns 99 rather than 10. Since the REPL and MCP worker catch evaluation errors and continue using that dictionary, a failed call silently corrupts subsequent program state. Restore the bindings in a finally block.

Checked 2026-10-29: still present on main; not touched by open PRs #3, #4 or #7.