docs: record what the subgroups plan built, and the six places it chose differently

All nine phases are built, one commit each. The phases are kept as written --
they are the reasoning, and the code reads better against the argument it came
from than against a summary of itself -- with a table of where the building
disagreed with the plan.

Six worth reading. `openCeremony` was never built, because a wrapper over two
repository calls the picker already makes would be a third name for one act.
`MarmotGroupCreation` is reached through the repository rather than called from a
view model, because view models here talk to repositories and managers take the
database. The guards' tests are in jvmTest rather than pure, because every refusal
reads the database and a pure version would test less. Phase 4 added two tags
rather than one, the second fixing a bug older than subgroups -- every robust
group has been arriving nameless on every device but its creator's. `stateFrom`
needed a third reader and a wrapper, because "tag present and unreadable" looks
identical to "absent" through a parser, and "refused" has to be distinguishable
from "none claimed". And Phase 8's two capability refusals short-circuited the
tests already written, which is how it came out that the fixtures had never had a
parent that could sign.

Two the plan got right and worth keeping if this is ever rewritten: the
key-package check moved to the picker on review, before a line was built, and it
is the difference between a subgroup failing in a second and failing after three
ceremonies; and the founding-roster rule has a test whose job is to fail the day
somebody adds the comparison that looks obviously missing.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/README.md

@@ -29,7 +29,7 @@ chronicle note is a phased plan that has not been built, and reads as the
 membership note's unanswered half: what a member who joins late can be given,
 and the one thing they cannot. The
 jvm-target note is unrelated to all of them: it is a build and packaging story.
-The subgroups note is a phased plan that has not been built; it assumes both
+The subgroups note is a phased plan that has been built; it assumes both
 shared-key notes and reads as the ceremony's second half — what a group does
 once it has a key, and what it can say about a group that does not yet.
 The Material Design note is a phased plan that has not been built, and is the