fix(sql): keep one cell per duplicate constant column

[EnRaiha]

Summary

SELECT nextval('a'), nextval('a') legally repeats an output name. The constant-result row is a JSON object keyed by column name, so the second cell overwrote the first and both wire columns rendered the last value. The payload and the output schema's lookup keys now use the same unique per-column keys every response encoder derives.

Red proof (the test fails on main without the fix)

────────────
     Summary [  10.891s] 1 test run: 0 passed, 1 failed, 10256 skipped
  TRY 2 FAIL [   4.311s] (1/1) nodedb::wire cases::sql_sequences::duplicate_constant_column_names_keep_their_own_cells
error: test run failed

Validation (local, on this branch)

Check	Result
the new test, with the fix	1 passed
related suites: sql_sequences + cell_keys + output_schema	53 passed
cargo fmt --all	no change
cargo clippy -p nodedb --all-targets --all-features --profile ci -- -D warnings	clean
repository hygiene gate (test registry, comment and commit hygiene, module contracts)	pass

One commit; touches sql_plan_convert/output_schema/build.rs, sql_plan_convert/set_ops.rs, and the new wire test only.

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[farhan-syah]

Approve.

Verified statically against the branch ref (no checkout):

Claim	Result
cargo fmt --all no change	Confirmed. rustfmt --check clean on all three files.
One commit, three files	Confirmed. 22a0f42, +59 / −13.
Test fails on main	Consistent with the code. On main both cells land under nextval, so the row holds one value and both columns read it.
Comment hygiene	Clean. No issue numbers, links, or pre-fix narration.

Layer is correct. Both producers of the constant-result shape (set_ops.rs payload, build.rs schema) now derive keys from cell_keys, the one function every encoder already reads through. No fallback, no second accepted shape.

Out of scope for this PR, tracked separately: the extended-query path (pgwire/handler/prepared/execute.rs) rebuilds its projection from the Describe fields with lookup_key = field name, and that projection overrides the planner schema (routing/execute.rs). SELECT nextval('s'), nextval('s') over Bind/Execute still collapses, as does SELECT w.id, b.id. That defect predates this PR.