[EPIC] Push pandas to the seams: views-frames migration for sample-bearing predictions #85

Problem

The FAO delivery is moving from point estimates to predictions-with-uncertainty (S samples per cell). As long as pandas sits on the data path, samples cannot propagate: a distribution becomes an object-dtype list-in-cell (the inherited PGMDataset representation) — the memory / encode-decode wall captured in register C-40 (and pipeline-core's own ~18 GB OOM, their #181). views-frames stores a distribution natively as a contiguous (N, S) float32 PredictionFrame.

This epic maps every pandas site in views-postprocessing and sequences the swap to native views-frames, one site at a time, each with a parity proof, disturbing cross-repo interconnectivity as little as possible.

Key enabling fact: the samples already arrive inside the forecast PGMDataset's object-dtype cells — they're collapsed to point estimates today only because unfao/frames.py hardcodes S=1 and the wires carry object-dtype parquet. So vpp can make its interior sample-aware unilaterally, while the two wire boundaries stay frozen until cross-repo coordination.

Why it matters

• It is the precondition for delivering uncertainty to FAO (FAO delivery path is point/DataFrame-based — won't carry rusty_bucket's pooled draws #45). At S=1 the pandas penalty is invisible; at S≈1000 it is the wall.
• It follows the maintainer's cross-repo strategy: push pandas to the seams of each repo, then swap one site at a time with parity.

The map — three concentric rings (grounded in file:line)

Ring 0 — already pandas-free (do not touch): views_postprocessing/delivery/{coverage,identity,observed_range,provenance}.py, unfao/gaul_schema.py, unfao/source_metadata.py.

Ring 1 — vpp-owned seams (UNILATERAL):

• unfao/extraction.py — the pandas→primitives seam (cells_of/months_of/drop_months_above/unmapped_cell_count).
• unfao/enrichment.py — pd.read_parquet(lookup) (:48) + .merge(...) (:117-119): a keyed metadata-attach join.
• unfao/frames.py — the unused views-frames adapter; _column_2d(...).reshape(-1,1) hardcodes S=1 (:85).
• scripts/build_gaul_lookup.py — offline lookup build (off the delivery path).

Ring 2 — forced / contract boundaries (CROSS-REPO-GATED):

• historical inbound read_dataframe → PGMDataset (unfao.py:58-64) + _append_metadata/_validate pandas ops — gated on pipeline-core DataFrame retirement (C-40; their S5 — The lookup declares its own version instead of inferring it (C-60) #186/Record the operator's correction decisions; put the FAO half to them formally #207/refactor(unfao): S3 — one representation seam; store metadata gets its own home #161/feat(delivery): S1 — one delivery path; refuse an incomplete launch config #159/feat(crafd): CRAF'd producer package — clone of unfao/ (per CLONING.md) #211).
• forecast inbound pd.read_parquet(io.BytesIO(...)) (unfao.py:125) — producer wire format.
• outbound df.to_parquet (unfao.py:298,309) → Appwrite — gated on the views-faoapi wire contract (FAO delivery path is point/DataFrame-based — won't carry rusty_bucket's pooled draws #45); faoapi already rebuilds views-frames at ingestion.

Desired end state

A forecast flows through vpp's interior as a PredictionFrame carrying (N, S), producing byte-identical deliveries to today on S=1 inputs (proving the interior is sample-ready) behind frozen wires. The final outbound hop that actually ships (N,S) to FAO (S6) and the historical inbound retirement (S7) are landed only when their cross-repo gates clear.

Scope

In: the 7 sequenced steps below (S1–S7), each independently shippable with a parity proof.
Out: Ring 0 (already pandas-free); pushing the enrichment metadata join into views-frames (S4 moves it off pandas, not into a frame — it is not frame algebra); changing the outbound format ahead of faoapi.

Stories (in sequence)

1. S1 — frames.py: generalize to_prediction_frame to S>1 [UNILATERAL] ← recommended first
2. S2 — extraction.py: frame-native sibling readers [UNILATERAL]
3. S3 — forecast "convert at the door" (internal PredictionFrame) [UNILATERAL]
4. S4 — enrichment.py: numpy/pyarrow keyed gather [UNILATERAL, low priority]
5. S5 — build_gaul_lookup.py: pyarrow-native build [UNILATERAL, lowest priority]
6. S6 — forecast outbound → arrow sample-frame wire [CROSS-REPO-GATED: faoapi #45 + C-40]
7. S7 — historical inbound read_dataframe/PGMDataset retirement [CROSS-REPO-GATED: pipeline-core C-40] — do not move yet

Dependency sequence

S1 ──► S2 ──► S3 ──► S6 (gated: faoapi #45)
        S4 (independent, low pri)
        S5 (independent, lowest pri)
S7 (gated: pipeline-core C-40) — independent of S1–S6; never blocks samples

Epic acceptance criteria

• S1–S3 merged: a forecast is represented internally as a (N,S) PredictionFrame; deliveries are byte-identical to today on S=1 inputs (parity).
• Each landed step carries a parity proof (build pandas → convert → assert arrays equal, and/or head-to-head old-vs-new) and keeps ruff + the full suite green.
• delivery/ remains pandas-free throughout (the design-contract test stays green).
• S6/S7 remain explicitly blocked until their gates (faoapi FAO delivery path is point/DataFrame-based — won't carry rusty_bucket's pooled draws #45 / pipeline-core C-40) clear; no unilateral wire change.
• Tracking issue complete.

Refs

Register C-40 (the pandas gate), #45 (the delivery-side draw carrier / faoapi wire), pipeline-core #186/#207/#161/#159/#211 (their DataFrame retirement). Parity pattern: tests/test_views_frames_conformance.py.

Issues

• Epic: [EPIC] Push pandas to the seams: views-frames migration for sample-bearing predictions #85
• S1 S1 — frames.py: generalize to_prediction_frame to S>1 (pandas→frames) #86 — frames.py → S>1 (keystone) [UNILATERAL]
• S2 S2 — extraction.py: frame-native sibling readers #87 — extraction frame-native readers [UNILATERAL]
• S3 S3 — forecast convert-at-the-door (internal PredictionFrame) #88 — forecast convert-at-the-door [UNILATERAL]
• S4 S4 — enrichment.py: numpy/pyarrow keyed gather (drop pandas merge) #89 — enrichment numpy gather [UNILATERAL, low pri]
• S5 S5 — build_gaul_lookup.py: pyarrow-native build #90 — build_gaul_lookup pyarrow [UNILATERAL, lowest pri]
• S6 S6 — forecast outbound → arrow sample-frame wire (ships (N,S) to FAO) #91 — outbound arrow sample-frame [GATED: faoapi #45 + C-40]
• S7 S7 — historical inbound read_dataframe/PGMDataset retirement #92 — historical inbound retirement [GATED: pipeline-core C-40]
• Tracking: #