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feat(sql): HAVING — post-aggregation filter (SQLR-52) (#161)

WHERE filters rows before grouping; HAVING filters groups after
aggregation. Closes the Phase 9e aggregates story.

Parser (src/sql/parser/select.rs):
- SelectQuery grows `having: Option<Expr>`, passed through raw from
  sqlparser like WHERE. parse_aggregate_call / AggregateFn::from_name
  exposed pub(crate) for the executor's HAVING lowering.
- HAVING without GROUP BY rejected with a typed NotImplemented (the
  degenerate single-group form SQLite allows isn't worth the executor
  branch in v0). HAVING + JOIN stays covered by the existing
  GROUP-BY-over-JOIN rejection (SQLR-6 is the follow-up).

Executor (src/sql/executor.rs):
- lower_having_expr rewrites aggregate calls in the HAVING tree to
  identifiers naming their output slot (SUM(salary) → "SUM(salary)"),
  registering hidden trailing projection slots for aggregates and
  GROUP BY keys referenced only in HAVING so aggregate_rows computes
  them alongside the visible ones.
- New GroupRowScope resolves those identifiers against the group's
  output row through the shared expression evaluator — comparisons,
  AND/OR/NOT, arithmetic, IS NULL, LIKE, IN all work, with the same
  NULL-as-false collapse WHERE applies (design-decisions §13).
- filter_groups_by_having runs after aggregation, before DISTINCT /
  ORDER BY / LIMIT; hidden slots are stripped after filtering.

Tests: +13 executor tests (612 → 625 in the engine suite): COUNT/SUM
thresholds, aggregate alias, aggregate-only-in-HAVING, group-key-only-
in-HAVING, compound AND, ORDER BY/LIMIT composition, all-groups-
excluded, NULL-aggregate collapse, lowercase call form, no-GROUP-BY
rejection, out-of-scope column rejection, all four JOIN flavors
rejected cleanly.

Docs: supported-sql.md (HAVING semantics section + syntax block),
sql-engine.md aggregation pipeline, roadmap.md shipped entry, README,
design-decisions §13 wording, web docs page / sql-ref / roadmap.

Co-authored-by: Claude F... (continued)

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