perf(gfql/polars): make the chain combine proportional to the traversal result

CHANGELOG.md

@@ -12,6 +12,7 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 - **GFQL secondary node property indexes (`create_index('node_prop', column=...)` / `g.gfql_index_node_props([...])`)**: a seed predicate on a NON-key column — `MATCH (m {id: 42})` where the graph's node id binding is some other column — previously cost a full node scan, because the registry only indexed the node-id binding and the CSR adjacencies. A property index is the same pay-as-you-go sidecar as the existing kinds: sorted distinct values over node **row positions** (CSR, so duplicate values are indexable), never reorders `.nodes`, fingerprint-validated so a `.nodes()` rebind is treated as absent (safe miss, never a wrong answer), engine-polymorphic (numpy host / cupy on-device), and policy-gated (`off`/`use`/`auto`/`force`). The seeded fixed-hop planner picks the **most selective** indexed scalar predicate in the seed filter using a free CSR-offset estimate, gathers those candidates, and applies every remaining predicate to them — so results are identical whether the index is present, absent, stale, or cost-gated out. `show_indexes()` lists property indexes; `drop_index('node_prop', column=...)` drops one. Only integer columns are indexable today (float NaN ordering, strings on cupy, and nulls all decline to the scan); widening that is additive. **Perf (dgx-spark, official LDBC SNB SF1, 3.18M nodes, warm median, value-identical 19-row result):** interactive-short IS7 `71.6 ms -> 19.5 ms` (**3.7x**), with a one-time `112 ms` build — the seed lookup itself goes from a `51.2 ms` scan to `0.096 ms`.
 
 ### Performance
+- **Native polars chain combine is proportional to the traversal result, not to the graph**: two graph-sized terms sat inside a combine whose answer is a handful of rows, and both are gone. (1) `_combine_edges` ran the prev/next endpoint gates for EVERY step, including the node steps whose edge frame is `g._edges.clear()` — zero rows. The eager combine skipped those, but the collect-once rewrite lazified the step frames and `.lazy()` erases the height, so the skip silently went dead. The cost lands on the side that is NOT empty: for the first step the gate's key side is the whole node table, and polars builds the hash table on that side before discovering the probe side has no rows (isolated: 6.99 ms for one such join at N=2M, and a chain pays one per node step). The pre-lazy row count is now recorded when the step frame is still eager and an empty step is dropped from the id union — it can contribute no ids, so the result is unchanged by construction. The skip keys on KNOWN-empty only; a frame that arrives already lazy reports no height and is planned normally. (2) The output node rows were materialized in TWO passes over the node table — one for the ids the steps kept, one more for the surviving edges' endpoints the first pass missed — then concatenated. The output node set is the UNION of those two id sides, so the ids are unioned first and the node table is read ONCE. The row-level `unique(subset=[node])` is preserved verbatim: those rows feed `how="left"` alias joins where a node table carrying the same id twice would multiply rows. Measured on synthetic LDBC-IS5-shaped graphs (one-row answer, polars-engine resident indexes), varying one dimension at a time: **at fixed E=2M, N=250k → 4M went 10.17 → 45.48 ms before and 7.60 → 17.17 ms after (2.65× at 4M, and the node-count slope is 3.7× flatter)**; the edge-count slope is unchanged, as expected for a node-side fix, with the constant ~6 ms lower. Parity: identical full frames (all columns, row order included) across 280 shape × graph combinations and 400 duplicate-node-id combinations, plus the 1003-case polars chain differential suite. Pinned by tests that assert the boundary rather than a wall clock: the node universe must not appear in the edge plan at all, and the node table must be read at most once per query.
 - **GFQL polars chain stops deduplicating semi-join key sides**: the native polars executor applied `.unique()` to every frame it fed into a `how="semi"` join. A semi-join emits a left row iff at least one matching right row exists, so duplicate keys can neither change which rows come back nor multiply them the way an inner join would — the deduplication was a full hash pass over the key column bought for no observable effect. On an unfiltered hop the key side **is** the node table, so this put **O(N) work inside a query whose answer is O(degree)**: the seeded single-hop plan built two such key frames per hop, each costing ~53 ms at 3.18M nodes — more than the rest of the query combined. The `.unique()` is now dropped everywhere the frame is provably a semi key side only (the `_semi` helper, the alias hop-window and next-edge endpoint gates, the two-hop fast path's endpoint gate, the `start_nodes` gate, the single-hop planner's id frames, and the index layer's `select_by_ids` polars branch — where the cuDF and pandas branches already used `isin` with no dedup, so the three engines now agree). It is deliberately **kept** on the alias frame that feeds a `how="left"` join, where duplicates genuinely would multiply rows, and the eager multi-hop loop is untouched (its frames also flow into concat/anti-join bookkeeping, a separate argument). Measured on a 3.18M-node / 14M-edge polars graph (LDBC SNB SF1-shaped), a seeded typed hop goes **127.4 → 57.1 ms (2.23×)** with identical row counts; at 1.75M edges **102.8 → 31.4 ms (3.28×)**, confirming the removed cost scales with node count, not edge count. Parity held across 380 differential comparisons covering duplicate node keys, null ids, dangling edges, duplicate `start_nodes`, and 11 traversal shapes; the gfql and chain suites show identical failure sets before and after. Pinned by tests that assert the boundary rather than the speed: duplicates reaching a semi key side must not change results or multiply rows, a dangling endpoint must still be excluded (the gate is load-bearing, not vacuous), and duplicate `start_nodes` must be inert.
 - **Seeded chain combine stops joining against the full frame when the intermediate is empty**: `_lean_prefilter_right` shrinks the big side of the combine's `how='left'` merge to the keys actually present on the left, but it declined to do so in the one case where shrinking is both maximally profitable and trivially correct — an **empty** left. A left merge keeps only the right rows that match, so a zero-row left yields a zero-row result whatever the right side holds; the merge was nonetheless materializing the whole graph-sized frame. It now hands back a zero-row slice of `right` (same columns and dtypes, so the merge still produces an identical schema). Measured: a single-node query whose 0-row intermediate was joined against 14M edges went **112.64 → 17.29 ms**. The shrink is used at exactly one call site, which is `how='left'`; a merge that retains unmatched right rows (`right`/`outer`) would NOT be safe to shrink this way, and the tests pin both directions — the empty-left case must return an empty result, and the non-empty cases must be byte-identical to the unshrunk merge.
 - **Native polars chain reuses the synthetic edge id as its stable row order**: when the executor had already added a synthetic edge-index column it also added a second, separate row-index column purely to restore input order at the end — two full `with_row_index` passes over the edge frame, and a redundant column carried through every intermediate. The existing synthetic id is already a contiguous, order-preserving row index, so it is now reused as the sort key and dropped once at the end. Only the pre-existing column is reused; when no synthetic id was added the separate order column is still created, so graphs that bring their own edge id are unaffected. Value-identical including row order (148.22 → 135.43 ms on a 3.18M-node / 14M-edge polars graph).