feat(example): a data room, end to end

[stormer78]

Track 1 of the delivery plan. Stacked on
#15 (the VAC/VDC types) — review and
merge that first; this targets that branch, and will retarget to main once it lands.

One example, one process, seven steps. cargo run --example data_room.

What it demonstrates rather than asserts

Step	Claim
1	A room is an entity — its own DID, issuing its own credentials
2–3	Joining is a two-party act: the VMC pair completes on Bob's acknowledgement, so a room cannot assert a membership nobody agreed to
4	Access is a chain that reaches the room. The host is not asked
5	An agent runs on strictly less than its human — read-only, four hours, audience-bound — and is refused write, which its parent holds
6	Removal actually removes: the epoch rotates, Bob's key cannot open what comes after. And forward-only is shown honestly — what he already held, he still holds
7	The host view: ciphertext, an epoch number, and nothing else

It closes by proving a relocated record does not open — the AAD binding, demonstrated
rather than described.

Sample output is in the commit; the interesting lines are agent write correctly refused: chain does not confer action write and Bob's key cannot open epoch 2 — removal actually removed.

Why it lives here

examples/ rather than a new repo: it demonstrates this library. Its crypto is a
dev-dependency, so nothing in the library's own graph encrypts — adding a cipher to a
credential crate for the sake of an example would be the wrong trade.

The one structural decision worth reviewing

The store sits behind a narrow RoomHost trait — put, get, list, epoch,
set_epoch — from the first commit. That is the difference between this being a
foundation and being throwaway: track 2 swaps the implementation for a VTC-backed one,
and that is only a swap if nothing above the trait reaches around it. Worth checking that
nothing does.

What it deliberately is not

Named in the module docs against what each gap does not prove, so the demo cannot quietly
be mistaken for the product:

• One symmetric key per epoch, not MLS — no post-compromise security, O(n) membership change
• did:key, not a witnessed did:webvh — no ownership transfer
• The attributed tier — the acting member is disclosed to the host, not proven in zero knowledge
• A BTreeMap, not a service — no availability, quota, or multi-party operation

What it does establish is that the credential model works end to end.