Observe the ActivityPods providers instead of asking them what they do

The previous commit treated this package like the polyfill: a provider whose
source is off limits, with the unknowns raised as questions. That was the wrong
regime. The no-read rule attaches to a provider that publishes an engagement —
this one publishes nothing at all, and for that case the protocol has a bounded
reconstruction, made once at adoption time and pinned. So the two questions are
answered here rather than asked, and one of them turned out to be about the Pod
rather than about the providers, which is not our concern.

The observation is now the specification: contract_activitypods-providers,
stamped at the commit it was read from. A record's id is the resource URI and it
is the only identifier, so the seam's two collapse into one. update fetches,
merges and PUTs, so a partial write is safe and the seam declares this backend
as patching.

What the reading also surfaced matters more than either question. Every
container resolution runs against the session's own WebID — in getList, in
create, in a list subscription alike. So this backend can only ever show a user
their own records: no other user's event is reachable, and an event's
participant count cannot be computed from participations the app cannot read.
That is the product, not a detail, so it is need 1 rather than a friction, and
it is phrased as what it is: the capability sits one layer down, exposed as a
WebID parameter, just not reachable through the data provider.

Four frictions are recorded with the workarounds, including the auth provider's
timer and listener that construction installs and nothing removes — which the
runtime backend switch would accumulate, so the interval goes off and the app
checks status itself.

.gitignore

@@ -43,3 +43,6 @@ playwright/.auth/
 storybook-static
 dist-staging/
 *.ngw
+
+# per-developer contract source access (never committed)
+.project/contracts.local.yaml