Pass 2: the planner chooses the segment grid

[zzylol]

Stack: #574 → #620 → #618 → #621 → #627 → #625 → #628 → #632 → #634 → #616 → #617 → #622 → #624 → #629 → #630 → #631 → #633 → #635 → #636 → #637

Problem

The shared-segment rule from #628 (share_window_segments, logical-optimizer Pass 2) offered only one grid: even segments whose width is the gcd of every window's lookback and offset. The planner never compared that grid with alternatives. For Example 3a, splitting only at the windows' own boundaries needs 4 segments instead of 5.

Changes (Q67)

• Two segmentations, each its own Stage 1 variant; Stage 3's cost picks one.
  • (a) Split only at the queries' distinct range boundaries. In 3a the boundaries are 0, 1, 2, 3 and 5 years back, which gives 4 segments, the oldest one 2 years wide.
  • (b) The even gcd grid. In 3a that is 5 one-year segments. It is offered only when it differs from (a); identical segmentations are deduplicated.
  • Labels: · shared segments ×4 and · shared segments ×5. A target's label lists uneven widths, for example Kll · 365d+365d+365d+730d segments. An even grid keeps the old label, 365d segments.
• WindowForm::Segments { unit_ms, ends }: the grid unit plus a bitmask of where each segment ends, so WindowForm stays Copy. WindowForm::widths gives the pane widths. tumbling_state and the W2 coverage proof (panes_cover_window) now take widths, so panes no longer need equal widths.
• Sharing::WindowSegments { segments } carries the number of distinct segments, which also keeps the two variants distinct.
• Accuracy is unchanged: every segment is sized for the group's strictest reader, and merging disjoint KLLs is exact.
• Stage 2 (materialization.rs): a SummaryMerge over panes of different widths is not a stream of panes, because no pane is an older pane shifted back. Its chain is therefore neither maintained at ingestion time nor kept. In 4a only the even grid has an ingestion-time option.
• Tests changed for the two grids: pattern_a_shares_four_boundary_or_five_one_year_segments (both forms, their ends masks and labels; 4 or 5 builds and 5 merges after composition), identical_segmentations_are_offered_once (new), stage1_a_shared_segments_serve_all_five (both candidates; each query's segments sum to its range), stage3_a_selects_the_cheaper_segment_grid (new), stage2_a_segments_are_built_once_for_all_consumers (4 and 5 segments, once and monthly), and the example4a_* devtools test (489 plans; only ×5 is maintained at ingestion time).
• tools/dag-viewer/examples.json: the 3a and 4a stories are updated. planner-layering-example1.json is unchanged (verified by regenerating it).

Costs of each segmentation (after #632's pricing)

Ex	×4 boundary split	×5 even 1-year grid	Selected
3a	P487 18,104.003889	P488 18,104.004444	P487 ×4
4a	P487 25.1444498 (query time; no ingestion-time option)	P488 25.1444506; P488-m1 (ingestion time) 1,536.68	P487 ×4

The two grids cost almost the same. Under #632's pricing each row of the scan lands in exactly one segment, so the range and build work is the same for both. The ×4 grid wins only because its merges read fewer states: 4+1+1+1+2 instead of 5+1+1+1+3. Shared input costs 25,112 (3a) and 34.878 (4a).

Test plan

• cargo fmt --all --check, cargo clippy --workspace --all-targets -D warnings
• cargo test --workspace: 1692 passed, 0 failed, 13 ignored (after the rebase on main d4869a7; was 1681 passed, 13 ignored)
• tools/dag-viewer: python3 -m unittest test_viewer: 25 OK
• Ran stage_pipeline for all six examples. Only the 3a and 4a selections change (to ×4).

Stacked on #632 (stack/windows-3-pricing).