fix(isthmus): elide redundant identity projections

[nielspardon]

Problem

Calcite wraps queries such as SELECT "a", "B" FROM foo GROUP BY a, b in a name-changing identity projection:

LogicalProject(a=[$0], B=[$1])
  LogicalAggregate(group=[{0, 1}])
    LogicalTableScan(table=[[FOO]])

The field references are $0, $1 in order — only the output names differ (b → B). SubstraitRelVisitor.visit(Project) turned every such projection into a Substrait Project, but Substrait carries output names on Plan.Root, not on the Project, so the node is pure redundancy. assertFullRoundTrip failed because the reverse conversion (Substrait → Calcite → Substrait) elides the identity projection, so root1 kept the project while root3 did not:

root1: Root{ Project{remap=[2,3], expressions=[$0,$1], input=Aggregate}, names=[a, B] }
root3: Root{ Aggregate, names=[a, B] }   ← project gone → assertEquals(root1, root3) fails

Change

Skip emitting a Project for an identity projection — all expressions are input refs, in order, with matching types, as reported by Calcite's own RexUtil.isIdentity. Because the check lives in visit(Project), it applies to both conversion legs, so the two sides converge to the project-free form and round-trip cleanly. Output names are unaffected: convert(RelRoot, ...) derives them from validatedRowType, independent of the projection.

Scope is limited to identity projections. Non-identity all-field-ref projections (permutations / column pruning) still emit a Project as before; that optimization is noted as future work in the retained TODO.

Tests

• New regression test NameRoundtripTest.roundTripNameChangingProjectionOverAggregate covering the exact query from the issue.
• The assertFullRoundTripWithIdentityProjectionWorkaround helper (whose Javadoc noted it should be removed long-term) is deleted; DdlRoundtripTest and DmlRoundtripTest now use the stronger plain assertFullRoundTrip.
• SubqueryPlanTest.existsNestedCorrelatedSubquery navigation updated: the inner UNIQUE (SELECT * ...) identity projection is now elided, matching how the outer EXISTS subquery was already represented. Downstream field-reference / steps-out assertions are unchanged, confirming the elision is semantically neutral.
• Full :isthmus:test, :isthmus:check (PMD + javadoc), and spotlessCheck pass.

Closes #371

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