feat(call): from_call adapter — discover + register remote ops (ADR-017 §3)

The #2 gap in alknet-call: discovers the remote peer's External operations
via services/list + services/schema and registers them in the connection's
Layer 2 overlay as FromCall-provenance leaves with forwarding handlers. The
discovery mechanism was already implemented in registry/discovery.rs;
from_call is the client-side consumer of that API.

src/client/from_call.rs:
- from_call(connection, FromCallConfig) -> Result<Vec<HandlerRegistration>,
  AdapterError>. Calls services/list then services/schema for each op,
  rebuilds OperationSpec from the schema JSON (parsing op_type, visibility,
  error_schemas, access_control), constructs a forwarding handler that calls
  the remote op via CallConnection::call(), and returns FromCall-provenance
  bundles (composition_authority: None, scoped_env: None, empty capabilities,
  remote_safe: false per ADR-028 §4).
- FromCallConfig { namespace_prefix: Option<String>, operation_filter:
  Option<HashSet<String>> } with builder methods.
- v1 defaults (two-way doors recorded in client-and-adapters.md):
  - error-on-collision (DC-3/OQ-28): applying the (possibly empty) prefix
    produces a name already seen -> AdapterError::Conflict, not silent
    overwrite.
  - auto-on-reconnect (DC-2/OQ-27): the overlay is per-connection (Layer 2,
    ADR-024), so re-import on reconnect is naturally scoped; the assembly
    layer calls from_call immediately after connect().
- Forwarding handler captures an Arc<CallConnection> and, on invocation,
  calls the remote op and returns its ResponseEnvelope. The
  parent_request_id participates in the cross-node abort cascade
  (ADR-016 §6) — if the parent is aborted, the cascade reaches this handler
  which sends call.aborted to the remote node; cross-node abort is
  transparent.
- Trust is transitive (recorded in spec): a from_call-imported op executes
  the remote node's code; scoped_env bounds which ops are reachable, not
  what they do.

OperationContext.internal is now pub (was pub(crate)) so downstream
consumers (assembly layer, integration tests) can construct contexts for
overlay-env dispatch.

Tests (207 lib + 2 integration):
- Unit: rebuild_spec name/prefix/op_type/visibility/error_schemas/acl;
  unknown op_type -> SchemaParse; missing op_type -> SchemaParse;
  FromCallConfig builder; from_call against a mock connection returns
  DiscoveryFailed (no transport); FromCall provenance + leaf fields + remote_safe false.
- Integration (tests/two_node_call.rs): from_call over a real QUIC loopback
  — CallClient connects, from_call discovers server/echo, registers the
  bundle in the overlay, and the forwarding handler round-trips an input
  through the overlay env to the remote op and back.

clippy + fmt + test all green.

Refs: tasks/call/client/from-call.md
Refs: docs/architecture/decisions/017-call-protocol-client-and-adapter-contract.md §3, §6
Refs: docs/architecture/crates/call/client-and-adapters.md §from_call

crates/alknet-call/src/registry/context.rs

@@ -19,7 +19,7 @@ pub struct OperationContext {
     pub env: Arc<dyn OperationEnv + Send + Sync>,
     pub abort_policy: AbortPolicy,
     pub deadline: Option<Instant>,
-    pub(crate) internal: bool,
+    pub internal: bool,
 }
 
 impl OperationContext {