[takashi-matsuyama]

What

Adds docs/spec/board.md, a reference for the shapes the experimental basou board reads and writes, as shipped in 0.71.0 (board_version 3, record_version 3):

• board.yaml: how it is read, every top-level key with what is required and its default, the stages and lanes, each of the ten kinds of measure (the files a measure of a repo sees, the glob dialect, regular expressions matched line by line, when a value is null), ratios, each of the nine kinds of observation with the value the guide asks for (the http value and its nulls included), the registry of components with the built-in markers and marker, the axis and the effort.
• The input of basou board record: its keys, the order it is checked in, observed, cells, prose, judged_by, axis_review, and what --json prints.
• A record: its keys, the declaration it keeps with its defaults filled in, and the order anomalies.
• The measurement: every section with what each null means (and which nulls mean something of their own), the not_found places, the methods, the difference from the previous record, and exactly what the digest covers.
• How a record of version 1 or 2 differs.

docs/spec/compatibility.md (under the list of experimental commands), docs/spec/cli-commands.md and docs/README.md point to it, and CHANGELOG.md gets an entry under ## Unreleased.

Why

Until now these shapes were described only in the changelog, in --help and in what basou board guide prints, so a board was declared by trial and error against the refusals of basou board measure. The page says at its top that every shape on it is outside every guarantee, as the command is, and may change at any release.

How it was checked

• Every statement was written from the source at 2c60650 (packages/core/src/board/, packages/cli/src/commands/board.ts).
• The page's board.yaml example and record input example were run through parseBoardDeclaration and parseRecordInput of the built @basou/core; both read.
• The refusals and defaults it quotes (unquoted stage ids, a missing or unknown board_version, a version 3 kind in a version 2 board, a group without a pattern, an observed_at without an offset, and others) were reproduced against the same build.
• The keys, methods and diff places it lists were compared with a real basou board measure --json.
• experimental.test.ts (which reads the table in compatibility.md), the CHANGELOG history guard, biome, the planning-identifier and absolute-path scans pass locally.