fix: report row stats for multi-table --table too

[yusukebe]

Summary

• emit the N rows extracted completeness note in the multi-table --table branch, aggregated: ax: note: 2 tables, 200 rows extracted, no empty fields
• count a row as empty only for fields its own table actually has (key union, per-row membership), so tables with different headers don't pollute each other's empty counts
• --where reporting now aggregates beforeWhere across tables as well

Why

Closes #61. rowStats() was only called when exactly one table matched; with two or more, stderr went completely silent — while the agent context explicitly teaches "treat that note as the completeness check — no extra verification probes needed". A guarantee that silently stops applying is the worst kind of gap. Reproduced with the issue's fixture and with table.wikitable on the Wikipedia GDP page (2 tables, cache-note only).

The multi-table output shape ([{headers, rows}, …]) is unchanged — only the stats survive it now.

Tests

One new CLI test covering: single-table note unchanged (no tables, prefix), aggregated multi-table counts, per-table empty-field attribution, and --where across tables. bun test: 159 pass in test/, tsc --noEmit and oxfmt --check clean.

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