refactor(core): typed credential registry replacing string-matched VC kinds (R19)

[stormer78]

Problem

handle_credential_issue string-matched "MembershipCredential" /
"EndorsementCredential" and stored each into one of two hard-coded
Option<serde_json::Value> fields on CommunityRecord. Adding a credential
type meant touching dispatch, storage, and the UI — five places that could
drift out of sync (R19).

Fix

A single typed registry:

• CredentialKind enum in openvtc-core/src/lib.rs, kept next to
  MessageType. It owns the vc_type (W3C VC type string), the persisted
  config_key, and activates_membership. CredentialKind::ALL is the one
  list dispatch / storage / UI iterate.
• Storage: CommunityRecord.credentials: BTreeMap<CredentialKind, Value>
  replaces the two Option fields.
• Dispatch: handle_credential_issue classifies via
  CredentialKind::from_credential and inserts — no per-kind branching.
• UI: the My Credentials panel and the presence flags iterate the registry.

Adding a kind is now one variant + its match arms; nothing in dispatch or UI
changes.

Config round-trip (no migration)

A deserialize-only CommunityRecordShadow (#[serde(from = ...)]) folds
pre-R19 flat membership_credential / role_credential fields into the map,
with new-format keys winning. The shadow keys the map by String on purpose:
an unrecognised kind written by a newer build is dropped with a warning rather
than making the whole config unloadable. CredentialKind serializes as its
config_key string so it is a valid JSON object key.

Router

No change needed: every credential kind shares one
credential-exchange/issue message type and DIDComm route, so a new kind
can't drift the route list.

Tests

• credential_kind_registry_is_self_consistent — from_credential /
  from_config_key round-trip for every ALL kind; unknowns → None.
• credential_issue_handles_every_registered_kind — dispatch stores + activates
  every registered kind generically (the acceptance criterion: a new kind
  touches the registry only).
• legacy_flat_credential_fields_load_into_registry — old configs load.
• typed_credentials_round_trip_and_tolerate_unknown — new format round-trips;
  unknown key dropped, not fatal.

Gate

cargo fmt --all, cargo clippy --workspace --all-targets -- -D warnings,
cargo test --workspace — all green. A code-reviewer pass on the diff found no
blockers (config round-trip, map-key serialization, and dispatch equivalence
all verified).

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