check: explain that XPath constraints can't compute values

[ako]

Sudoku-app finding #8: retrieve … where [Seq = $Game/MoveSeq + 1] fails with a bare mismatched input '+' expecting ']', with no hint about the actual cause.

This is a Mendix limitation, not an mxcli bug: XPath constraints take a literal / token / variable / path on the value side, never an arithmetic expression. So adding grammar support would be the wrong fix — it would let through XPath that mxbuild then rejects. The right fix (matching the finding's own framing — "the failure is a parse error rather than a message about XPath") is a clear diagnostic.

Fix

Add an error hint keyed on mismatched input '<+|*|div|mod>' expecting ']'. The expecting ']' shape only occurs inside a bracketed constraint, so the signal is precise — a normal-expression arithmetic error (e.g. expecting ';') does not trigger it. The hint names the limitation and shows the workaround:

Mendix XPath constraints cannot compute values (no +, -, *, div, mod).
Compute the value into a variable first, then compare against it:
  $Next = $Game/MoveSeq + 1;
  retrieve $M from Mod.Move where [Seq = $Next] limit 1;   (correct)

Tests & docs

• Visitor test: the hint fires for +/*/div/mod inside a constraint and not for a normal-expression arithmetic error.
• Negative-test example f8-xpath-arithmetic.fail.mdl.
• xpath-constraints.md gains an "arithmetic on value" row + workaround note; fix-issue.md symptom row.

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