The graph keyed formulas by a string joining sheet name, column and row
with a raw NUL separator, which put a literal 0x00 byte in the source.
Git then treated the file as binary, so it had no readable diff or blame.
The spill tracker in excel.dart already moved to a (name, row, column)
record; this applies the same shape here, which is also cheaper than
building a string per formula per recalculation.

The writer test deliberately feeds U+0000 through U+0002 into a cell to
prove they are stripped on write. It now spells them as escapes, which is
identical at runtime and keeps that file readable too.

No tracked text file in the repo contains a NUL byte any more.