`feat` Aggregations, output aliases, and traversed projection on the materialization plan

[0x054]

What to build

The remainder of superseded epic #225, downstream of the materialization-plan substrate (#277): aggregations (sum/avg/min/max, GROUP BY) materializing into projected records, plus the two projection extensions deliberately deferred from #277 because they share one design — output aliases and traversed projection (select reaching across a relation, INNER-narrowing per ADR-0006).

Design note carried from the 2026-07-10 session: aliases name output record fields, not join identities (no conflict with ADR-0006's no-alias rule for joins). Leading surface candidate: the dict-returning lambda — select(lambda t: {"id": t.id, "account_name": t.account.name, "total": ...}) — keys are field names; one lambda, no new proxy API, generalizes to aggregate expressions. t.account.id as "x" is a Python SyntaxError and cannot be a surface. The record plan shape already reserves name-vs-column separation, per-field path, and expr (ADR-0007), so this issue extends the section without reshaping it. Unaliased traversed projection must resolve name collisions (t.id vs t.account.id) with a build-time error pointing at the alias form.

Acceptance criteria

• Aggregate values land in projected records (Rows[Row]) with user-named fields, both backends; GROUP BY composes with where (traversal included).
• Output-alias surface decided and shipped (dict-lambda is the candidate to beat); traversed projection works at any depth with INNER-narrowing semantics and left_join interplay pinned.
• Same-name collision in an unaliased traversed projection raises at build time naming the alias form.
• Materialization-plan expr/path field shapes exercised end-to-end; golden vectors + Rust round-trip pins; static fixtures extended.

Blocked by

• feat Partial selects 2/4: select(lambda) → Rows[Row] — the tracer bullet #279