feat(isthmus): map RIGHTSHIFT to Substrait shift_right

[nielspardon]

What

Adds a mapping so a right-shift can be converted to the Substrait shift_right function. Calcite defines the << operator (BIT_LEFT_SHIFT) and the LEFTSHIFT function — both already mapped to Substrait shift_left — but has no right-shift counterpart at all. This PR defines an Isthmus RIGHTSHIFT(value, shift) SqlFunction (mirroring Calcite's LEFTSHIFT), registers it in SubstraitOperatorTable, and maps it to shift_right.

Why

shift_right exists in functions_arithmetic.yaml (i32/i64, same shape as shift_left) but was unreachable from SQL because no Calcite operator produced it.

Scope

• Covers the function-call spelling RIGHTSHIFT(x, n). Round-trips to Substrait shift_right and back.
• The >> infix operator is intentionally out of scope. Calcite 1.42 has no >> token, no SqlKind, and no operator (only <<). Supporting >> requires isthmus to generate its own parser: Calcite exposes the needed extension hooks (binaryOperatorsTokens + extraBinaryExpressions, the same mechanism calcite-babel uses for the :: infix cast), but isthmus currently consumes the prebuilt SqlDdlParserImpl and has no parser codegen. Tracking that as a separate follow-up; the cleanest long-term fix is likely to upstream BIT_RIGHT_SHIFT into Calcite core (a genuine gap, given it already has <<), after which this reduces to a one-line mapping.

Testing

• New round-trip test in ArithmeticFunctionTest (i8/i16/i32/i64), symmetric to the existing leftshift test.
• Full :isthmus:test suite passes locally.

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