feat(board): split the age column, so a silent repo cannot hide behind a fresh plan

ALDER carried two readings depending on which branch a repo fell into: STATE.md
mtime for repos that had one, last-commit age for repos that did not. The
consequence was one-sided -- a repo WITH a STATE.md reported only how recently
its plan was touched, so one that had not committed in a year looked exactly as
alive as one worked on this morning, and no column contradicted it.

Now each column means one thing. ALDER is the plan's age and is blank where
there is no plan; SISTE is the last commit, read for every repo, and blank where
there are no commits -- a day count there would be a fabricated reading, which
is what the old -1d was.

The four ranking rules stay four: the new column is evidence, never sort input.
The sort key was already a separate field from the displayed age, so bucket 5
keeps ordering by commit age while showing nothing in ALDER.

The fixture tree was missing the only case that discriminates -- a repo with a
STATE.md AND an old last commit -- so the checks land with it, plus its mirror
image (old plan, fresh commit) to pin that the two ages read independently and
that neither became the sort key. Backdating sets GIT_COMMITTER_DATE, not
--date=: %ct is the committer date, and an author-only backdate would have left
the red test failing for the wrong reason.

board-selftest: 30 -> 36 checks.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QS6gYYgYgJAz7VjcFHEzbb

README.md

@@ -132,7 +132,7 @@ Note that raising the inbox's priority (Rule 7) deliberately does **not** widen
 ## Development
 
     bash scripts/coord-selftest.sh    # 136 checks against a throwaway mailbox
-    bash scripts/board-selftest.sh    # 30 checks against a throwaway repo tree
+    bash scripts/board-selftest.sh    # 36 checks against a throwaway repo tree
     bash scripts/route-selftest.sh    # 50 checks, incl. the route->board round trip
     npm test                          # all three selftests via node --test