feat(eda): capítulo MISSINGNESS — patrones de datos faltantes (co-ocurrencia + MCAR/MAR)

Añade el capítulo `missingness` al motor AutomaticEDA, complemento natural de
`calidad`: donde calidad reporta cuánto falta por columna, este capítulo analiza
el PATRÓN de los nulos — dónde faltan y si las columnas faltan juntas
(co-ocurrencia de ausencias), la señal que distingue MCAR de MAR antes de imputar.

Capítulo (`chapters/missingness.py`), registrado en `chapters_registry.py` justo
tras `calidad`:
- Resumen global: % de celdas faltantes, columnas con nulos, filas completas vs
  incompletas.
- Ranking por columna (tabla + barras horizontales).
- Co-ocurrencia: correlación de las máscaras is-null entre columnas (heatmap +
  tabla de los pares que co-faltan, con co-faltantes y Jaccard).
- Patrones de fila más frecuentes (estilo matriz de missingno).
- Lectura MCAR/MAR exploratoria (heurística por correlación/solape de ausencias,
  no confirmatoria), que cita la evidencia concreta.
- Términos de glosario clicables: missingness, MCAR, MAR.

La máscara is-null por fila de TODAS las columnas (numéricas y categóricas) se
construye con un push-down DuckDB sobre ctx['db_path']/table (mismo patrón que el
capítulo agregación), con fallback a ctx['raw_numeric'] cuando no hay BD. Activa
solo si la tabla tiene nulos; si no, devuelve None.

Funciones nuevas del grupo `eda` (dominio datascience):
- extract_null_mask (impura): máscara is-null por fila vía query_fn.
- missingness_overview (pura): resumen global + filas completas/incompletas.
- missingness_correlation (pura): correlación de ausencias + pares + Jaccard,
  reutiliza pearson.
- missingness_row_patterns (pura): patrones de fila más comunes.
- missingness_corr_heatmap_figure / missingness_rank_bar_figure (impuras): figuras.

Verificado: EDA de titanic genera el capítulo en PDF + PPTX + MD con Cabin 77.1%,
Age 19.9% y la co-ocurrencia Age↔Cabin (158 filas). Suite completa de AutomaticEDA
+ render_automatic_eda en verde (125 passed); tests por función y por capítulo;
fn index sin error.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

python/functions/datascience/missingness_overview.py

@@ -0,0 +1,116 @@
+"""Pure EDA helper: dataset-level missingness overview from a 0/1 null mask.
+
+Part of the `eda` capability group. Consumes a per-column null mask
+(``{col_name: [int 0/1, ...]}`` aligned by row, ``1`` = value is missing,
+``0`` = value is present) and derives dataset-wide missingness metrics: cell
+count and percentage of missing data, how many columns carry any null, and how
+many rows are complete vs. incomplete.
+
+Dict-no-throw style of the `eda` group: it NEVER raises. A non-dict, an empty
+dict, malformed columns, ragged lists or non-int cell values all degrade
+gracefully to the zero/contract output. Stdlib only.
+
+Ragged-length policy: columns are allowed to have different lengths. ``n_rows``
+is the **maximum** column length; positions that don't exist in a shorter
+column are treated as present (``0``). This keeps the ``n_rows * n_cols`` cell
+grid well defined without dropping rows.
+"""
+
+
+def _is_missing(value) -> int:
+    """Return ``1`` iff ``value`` denotes a missing cell, else ``0``.
+
+    Only an exact equality to ``1`` (covers ``int`` ``1`` and ``float`` ``1.0``)
+    counts as missing. ``None``, ``0``, strings and any other value are treated
+    as present. The comparison cannot raise for standard inputs.
+    """
+    try:
+        return 1 if value == 1 else 0
+    except Exception:
+        return 0
+
+
+def missingness_overview(null_mask) -> dict:
+    """Summarize dataset-level missingness from a 0/1 null mask.
+
+    Args:
+        null_mask: Dict ``{col_name: [int 0/1, ...]}`` where each list is aligned
+            by row (``1`` = missing, ``0`` = present). Lists are normally all the
+            same length (= number of rows). Defensive: a non-dict or empty dict
+            returns the all-zero contract; non-list columns are treated as empty;
+            ragged lists are aligned to the maximum length, padding the missing
+            tail of shorter columns as present (``0``); ``None`` / non-int cells
+            count as present.
+
+    Returns:
+        Dict with exactly these keys, all always present (the function never
+        raises): ``n_rows``, ``n_cols``, ``n_cols_with_null``,
+        ``n_missing_cells``, ``missing_cell_pct`` (0-100), ``complete_rows``,
+        ``incomplete_rows``, ``complete_pct`` (0-100), ``incomplete_pct``
+        (0-100). Percentages are ``0.0`` when the denominator is zero (no
+        ``ZeroDivisionError``).
+    """
+    zero = {
+        "n_rows": 0,
+        "n_cols": 0,
+        "n_cols_with_null": 0,
+        "n_missing_cells": 0,
+        "missing_cell_pct": 0.0,
+        "complete_rows": 0,
+        "incomplete_rows": 0,
+        "complete_pct": 0.0,
+        "incomplete_pct": 0.0,
+    }
+
+    if not isinstance(null_mask, dict) or not null_mask:
+        return dict(zero)
+
+    # Normalize every column to a list; non-list columns become empty.
+    cols = {}
+    for name, seq in null_mask.items():
+        cols[name] = seq if isinstance(seq, (list, tuple)) else []
+
+    n_cols = len(cols)
+    lengths = [len(seq) for seq in cols.values()]
+    n_rows = max(lengths) if lengths else 0
+
+    if n_rows == 0:
+        # Columns exist but carry no rows: everything zero except n_cols.
+        out = dict(zero)
+        out["n_cols"] = n_cols
+        return out
+
+    n_missing_cells = 0
+    n_cols_with_null = 0
+    row_has_missing = [False] * n_rows
+
+    for seq in cols.values():
+        col_len = len(seq)
+        col_has_null = False
+        for r in range(n_rows):
+            if r < col_len and _is_missing(seq[r]):
+                n_missing_cells += 1
+                row_has_missing[r] = True
+                col_has_null = True
+        if col_has_null:
+            n_cols_with_null += 1
+
+    incomplete_rows = sum(1 for flag in row_has_missing if flag)
+    complete_rows = n_rows - incomplete_rows
+
+    total_cells = n_rows * n_cols
+    missing_cell_pct = (n_missing_cells / total_cells * 100.0) if total_cells else 0.0
+    complete_pct = complete_rows / n_rows * 100.0
+    incomplete_pct = incomplete_rows / n_rows * 100.0
+
+    return {
+        "n_rows": n_rows,
+        "n_cols": n_cols,
+        "n_cols_with_null": n_cols_with_null,
+        "n_missing_cells": n_missing_cells,
+        "missing_cell_pct": missing_cell_pct,
+        "complete_rows": complete_rows,
+        "incomplete_rows": incomplete_rows,
+        "complete_pct": complete_pct,
+        "incomplete_pct": incomplete_pct,
+    }