Opening an .xlsm and saving it silently stripped xl/vbaProject.bin, turning a
macro-enabled workbook into a macro-free one — data loss for anyone whose work
depends on macros. GridCraft still does not run VBA, but it now carries the
project through unchanged.
- model: `Workbook.vba_project: Option<Vec<u8>>` (base64 in JSON via a new
`b64::opt` helper). `None` = no macros.
- xlsx read: capture the `vbaProject` relationship's bytes instead of warning
and dropping them; warn that macros are kept but not executed.
- xlsx write: when present, emit `xl/vbaProject.bin` byte-identical, add the
`.../relationships/vbaProject` relationship, switch the main part to the
macro-enabled content type (`...sheet.macroEnabled.main+xml`) and add the
`application/vnd.ms-office.vbaProject` override.
- engine: `view.macros` lists the preserved project alongside GridCraft's own
scripts (`{"kind":"vba","executed":false}`), so callers can see it is there.
- ROADMAP: scope now "preserve, don't execute" (was "out of scope").
Tests: round-trip byte-identical blob + correct content types/relationship;
capture from a foreign-produced .xlsm; a macro-free workbook stays clean.
justinjohn0306