docs(project): record the M3.10a delivery and name the next slice

Records how version six was delivered, prunes the planning material the
slice consumed, and sets the next action to M3.10b — the version-six
execution model and its recorded transition trace, issue #153 rebound
from version five.

The execution model comes before the C++ codec, and the order is the
repository's own lesson rather than a preference: M3.9a implemented a
codec and M3.9b found two implementations agreeing perfectly about a
message neither could construct, because a codec never asks where a
transaction gets its arguments.

Five paragraphs of superseded guidance are pruned: the version-five C++
notes, the retain-or-replace question the new text answers, the M3.9e
paragraph that version six rebinds, and a duplicated harness note. The
keep-accepted list now records that version four's package is
load-bearing three ways and version three's twice.

Also records one local hazard that cost time and will again: stale
__pycache__ made a reverted mutation appear to still fail and a real
failure appear to pass. Hosted runners start clean, so it is local only.

Refs #169

Author: kaikisegfault