Import upstream v0.16.24, stripped

Upstream commit: af37a234981722493b74623a983581691d2b70b6
Enterprise-only files removed or emptied: 63
Enterprise-only snippets removed: 118 in 50 files
Dangling module declarations removed: 5
Edits turning enterprise off: 25
Third-party code: 14 files, 0 not in THIRD-PARTY.md
Renamed identifiers: 62 in 18 files
Verification: clean

The same Enterprise footprint as v0.16.23. The build check fails only on
tests/src/directory/issuer.rs, unchanged since v0.16.23: it calls a helper
from upstream's Enterprise-only OIDC test, and tests issuer-based directory
routing, an Enterprise feature. main has never carried it.

crates/jmap-proto/src/request/reference.rs

@@ -105,6 +105,12 @@ impl<V: Default> Default for MaybeResultReference<V> {
     }
 }
 
+impl<V: FromStr> Default for MaybeIdReference<V> {
+    fn default() -> Self {
+        MaybeIdReference::Invalid(String::new())
+    }
+}
+
 impl<T: Default> MaybeResultReference<T> {
     pub fn unwrap(self) -> T {
         match self {