feat!: a VDC is not a bearer credential; upgrade ordering; digest wording

[stormer78]

Closes #23 and #22, plus a stale-documentation fix found while checking them.

Important

API-breaking despite the patch version. delegation::verify_chain takes a new
argument. 0.9.1 was chosen deliberately — 0.9.0 published hours earlier with no known
consumers — but Cargo treats 0.9.1 as semver-compatible with 0.9.0, so anyone on
dtg-credentials = "0.9" picks this up on a routine cargo update. They get a compile
error naming the new parameter rather than a silent behaviour change. Flipping to
0.10.0 is a one-line change in Cargo.toml plus the changelog heading if you'd
rather force the opt-in.

1. A VDC is not a bearer credential (#23) — breaking

delegation::verify_chain now takes presenter: &str and requires the leaf to appoint
it, refusing with the new DelegationError::NotTheDelegate. Nothing else about chain
verification changed.

#21 did this for the VAC and left the VDC alone. The asymmetry was the whole gap:
VerifiedDelegation.delegate was returned, so a caller could compare it, but nothing
in the signature obliged them to — exactly the shape the VAC was in before #21, where
two consumers each found the omission independently and patched around it.

Delegation is the sharper of the two cases. A captured VAC replays whatever it confers;
a captured VDC replays as somebody, and every act it carries is attributed to the
principal.

Three tests cover it: a captured VDC refused for its captor, the principal refused for
its own delegation, and an intermediate delegate refused for a chain below it — that
last one pinning that only the leaf's delegate is asked for anything, which is what
keeps re-delegation working.

2. Upgrade ordering (#22)

New Upgrading section in the README: verifiers before issuers, with a table covering
0.7.0, 0.8.0 and 0.9.1.

One note on scope. #22 suggested a hint in BrokenLink when named decodes as a digest
and presented does not. That cannot help the reported case: the confusing message is
emitted by the 0.6 verifier, and nothing shipped here changes what 0.6 prints. The
direction this crate does control was already correct — an old credential reaching a new
verifier fails with InvalidDigest, which names the WD01 sha256:<hex> form explicitly.
So the ordering guidance is the whole of the fix, which is what the issue itself expected.

3. BrokenLink documented its fields as identifiers

Found while checking #22, and not in either issue. AuthorityError::BrokenLink described
named and presented as "the id the link points at" / "the id of the credential
actually presented". Both have carried digests since 0.7.0.

That stale wording is plausibly part of why the skew in #22 reads as a mismatched chain
to whoever goes looking — the error prints two values and the docs told you they were
identifiers. Both now say digestMultibase, and the variant points at the upgrade
ordering. DelegationError::BrokenLink documented its fields not at all, and now matches.

One correction to #23

The issue notes the 0.7.0 changelog "already claims a VAC and a VDC are both non-bearer,
and that is only half true until this lands." In context (CHANGELOG.md, 0.7.0) that
sentence sits inside a list of what 0.7.0 explicitly did not implement, describing
upstream PR #41 — it was accurate when written, and 0.8.0 doesn't overclaim either. The
gap was real; no changelog needed correcting, so none was.

Notes

• 124 tests, up from 121. cargo clippy --all-targets and cargo fmt --check clean.
• The data_room example now presents the VDC as the scheduler, and demonstrates the
  same VDC being refused for anyone else. Verified by running it.
• @stormer78 — the ordering guidance you were waiting on is in the README section above,
  and it is verifiers first for every breaking release so far. The VTI bump from 0.6
  now crosses three of them.

Refs: #22
Refs: #23

[affinidi-appsecurity-bot]

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