criteria: add where_substitutions to read the substitutions of a where criterion

[m-messer]

Stacked on #6: merge that first, and this PR's base will then switch to main.

Moves the node-reading half of compareExpressions' parse_where_substitutions into lambdafeedback.criteria.

Changes

• where_substitutions(substitutions) in the new where.py takes the second child of a WHERE node (one EQUALITY, or a SEPARATOR of them) and returns [(name, expression), ...] with the expressions as text.
• The caller parses the expressions, so criteria still doesn't depend on expression_parsing.
• Behaviour change (CHANGELOG): the grammar also accepts other relations after where (where x > 2, where x proportional to y).
  • compareExpressions' where_substitution_nodes returned the node itself for these instead of a list; this raises CriteriaSyntaxError.
  • A SEPARATOR that mixes in such a relation (where a = 2; x > 3) used to be read as substitutions, and also raises now.

Downstream
compareExpressions keeps parsing the expression text itself, and should catch CriteriaSyntaxError where it previously handled a non-list result.

Verification

• ruff, format and mypy are clean, and all four test suites pass.
• New tests cover one, two and three substitutions, and each unsupported relation.

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