v0.2.0

The format discovery engine, a direct Arrow path for Parquet, and a
per-model format selector in ldf-mcp. The specification stays v0.1;
two encoder bugs are fixed against it.

Added

• ldf-mcp --model ID (in proxy, clickhouse and files): each result
  goes out as LDF only when measured evidence for that exact model on data
  of the same shape shows no accuracy loss against JSON (at least 50
  questions, no more than 3 points below, no significant loss, no more
  rows than measured) and LDF is at least 5 percent smaller; otherwise
  JSON, with the reason on stderr. A model without evidence gets JSON
  throughout and no LDF primer. --evidence FILE replaces the built-in
  evidence. Without --model nothing changes.
• ldf-select, a new crate with the selector's rule and the evidence
  table built in, shared by ldf-mcp and ldf-discover.
• ldf::arrow (feature arrow): encodes Arrow RecordBatches directly,
  byte-identical to the record path, 1.9x to 3.2x faster in block form at
  1M rows and 1.4x to 4.3x less memory. ldf-mcp convert uses it for
  Parquet and falls back to the record path for lists of structs.
  ldf_data::input::ColumnReader is public for it.
• ldf-discover (not published): a parameterized encoder whose layout
  choices are knobs, a decoder for every point, Stage A token search over
  the space, and the select command.
• Evaluation: four new task kinds, more questions per kind, held-out
  fixtures, a bare answer outcome, Stage B and the DeepSeek sparse sweep
  (reports under docs/reports/), eval/analysis/evidence.py for the
  selector's evidence, and eval/bench_mcp.py for end-to-end MCP timing.

Fixed

• An object field that is null in some rows decoded as absent; such a
  field is now a j column (spec 7.1).
• A bare string starting with @ as the first cell of a row was read back
  as a header; it is now quoted (spec 5). LDF output on the twelve medium
  fixtures is byte-identical before and after both fixes.