v0.2.0 - byte-faithful writes and content-hash guards

[0.2.0] - 2026-10-15

Fidelity and guard hardening: a notebook now survives a write untouched except
for the edit you asked for, and every mutating tool can be guarded by a content
hash rather than a timestamp. Ported back from the Python CLI skill that had
grown these properties first.

Added

• Layout-preserving serialization. lib/nb/style.js probes the layout a
  notebook already uses — JSON indent (0/1/2/4), CRLF vs LF, and whether the
  file ends with a newline — and writes it back in that same layout. Cells whose
  source is a plain string stay a string; keys that were absent
  (outputs, execution_count, metadata) are not invented. A file that is
  read and written back unmodified is byte-identical, which makes the plugin
  safe to point at notebooks produced by JupyterLab, nbformat.writes, or hand
  editing. Known limits: tab indentation, \uXXXX-escaped non-ASCII (Python's
  ensure_ascii=True), and more than one trailing newline are normalized to the
  tool's defaults.
• expected_file_sha256 on all four mutating tools. expected_file_mtime_ms
  cannot see a rewrite that happens inside the same millisecond; the content
  hash can. notebook_read and notebook_list_cells now return file_sha256
  so the value never has to be guessed.
• expected_source_sha256 on notebook_insert_cell (its anchor cell) and
  notebook_clear_outputs (its target cell). Every mutating tool now accepts
  the same three guards.
• Deterministic ids for notebooks too old to store them. A missing cell id
  is derived from sha256("v1|<ordinal>|<source>") instead of random bytes, so
  the same cell gets the same id on every load, across processes.
• Structure validation before every write. The save path runs the deep
  audit and refuses to write a notebook that fails it — cell ids must match
  ^[A-Za-z0-9_-]{1,64}$ and be unique, metadata must be an object when
  present, a code cell's outputs must be an array when present, and every
  element of a source array must be a string. notebook_validate reports the
  same deep problems.
• test/fidelity.test.js — byte-for-byte round trips across six file layouts,
  deterministic id behavior at nbformat 4.4 and 4.5, guard coverage for all four
  writers, append-only behavior for an unedited cell, and the documented
  non-preservable cases. Suite: 61 → 75 assertions.

Changed

• Writes no longer reformat the whole file. The old save path rebuilt the
  document and forced a 1-space indent, which reformatted every notebook it
  touched (and silently dropped unknown root keys).
• nbformat_minor is no longer bumped. Backfilling an id never changes the
  declared notebook format.

Fixed

• README and CHANGELOG claimed id backfill "bumps nbformat_minor to 5". No
  code path ever did that; the documentation now matches the implementation.
• A notebook that is valid enough to inspect but structurally broken is now
  reported by notebook_validate and refused by the writers, instead of being
  silently loaded and rewritten.