pyramid: two-phase iterative build (finest from source, then rollups) (#138)

* spec: two-phase iterative hex pyramid build to bound build-time memory

Replace the single 7-branch UNION ALL COPY with one finest-from-source
COPY followed by iterative parent rollups from the previous resolution.
Bounds working memory by output cardinality per step instead of by full
source size, so global high-cardinality datasets (GBIF, irrecoverable
carbon) build without OOM.

Also adjusts .gitignore to track committed specs under
docs/superpowers/specs/ while continuing to ignore the rest of
docs/superpowers/ (matches the precedent set by the existing 2026-04
specs).

* plan: implementation steps for two-phase iterative hex pyramid build

* tiles: bump layout version to v3-iterative for two-phase pyramid

* pyramid: two-phase iterative build (finest from source, then rollups)

* pyramid: rename AVG internal weight column; fix docstring; cover multi-col SUM

- Rename AVG-mode internal column count → __pyramid_weight to avoid collision
  when a user names their value column "count"
- Fix register_hex_tiles docstring: "Other aggs" bullet now correctly states
  aggregation happens at every resolution including finest (not passed-through raw)
- Add test_sum_mode_with_multiple_value_columns covering multi-value-column SUM

* test: end-to-end mathematical invariants for two-phase pyramid

* test: AVG weighted-mean fixture must span two h5 cells

Original fixture placed all five points within ~100m, so they collapsed
to a single h5 cell at the finest level. Phase 2 then saw one child per
parent, where SUM(v*w)/SUM(w) and an incorrect AVG(v) both yield the
same answer. The test would have passed even with the unweighted bug
it was meant to catch.

Spread the two clusters to (37.0, -122.0) and (37.2, -122.0) — distinct
h5 cells, same h4 parent — and add a sanity assertion that the finest
level really has two distinct h5 children before checking parent AVG.

Author: cboettig

.gitignore

@@ -8,7 +8,9 @@
 # Claude Code / superpowers tooling
 .claude/
 memory/
-docs/superpowers/
+docs/superpowers/*
+!docs/superpowers/specs/
+!docs/superpowers/plans/
 
 # Byte-compiled / optimized / DLL files
 __pycache__/