The single point through which every mutation becomes a filesystem
operation: read fresh from disk (strict byte-faithful UTF-8), refuse
readable-but-uneditable frontmatter shapes, apply the edit through the
span engine, stamp modified / clear modified-by, then temp-file+rename
atomically. Renumber-visible-children is the sole minimal-touch
exception, tombstones untouched. Structured BoardWriteError carries
operation + path + reason for the future banner surface.
Alongside, three edges the card surfaced:
- The loader now decodes byte-faithfully too, so a BOM'd or non-UTF-8
index.md is rejected at load per the encoding contract, instead of
loading via NSString's silent BOM strip and then refusing every write.
- An appended frontmatter line adopts the file's prevailing line ending
(a CRLF file stays uniformly CRLF when a stamp first lands in it).
- Empty-but-not-blank frontmatter ({}, null, ~) is detected as
uneditable — appending after it would be unparseable YAML.
30 new unit tests; 176 total green.
Claude-Session: https://claude.ai/code/session_018BjQRYBR6jQja3jCRi5S3A
