We want to endogenously tag TP53 with mCherry in HEK293T so we can do
live-cell imaging of p53 dynamics under genotoxic stress. C-terminal fusion,
SpCas9, HDR donor.

Need:
- guides at the TP53 stop codon, cross-validated across at least 2 tools
  (had a previous design fail in the lab because of a pseudogene match
  none of us caught with a single tool)
- HDR donor with ~800 bp homology arms each side, GS linker before mCherry,
  PAM-blocking silent mutations so the donor doesn't get re-cut
- junction validation primers for genotyping (5' and 3' anchored outside
  the homology arms) plus an internal Sanger primer pair across the insert
- full off-target report for the top guides

Cell line is HEK293T. Reference TP53 NM_000546.6 on GRCh38. The mCherry
should be codon-optimized for human and contain no internal BsaI / BsmBI
sites since we may re-clone it later.
