Add OpenSearch search backend with hybrid neural+BM25 support

Extract a SearchBackend trait from the existing RocksDB search code and
add an OpenSearch implementation supporting cross-room search, relevance
ranking, fuzzy matching, English stemming, and optional hybrid
neural+BM25 semantic search using sentence-transformers.

Fix macOS build by gating RLIMIT_NPROC and getrusage to supported
platforms.

src/core/utils/sys/limits.rs

@@ -28,7 +28,12 @@ pub fn maximize_fd_limit() -> Result {
 #[cfg(not(unix))]
 pub fn maximize_fd_limit() -> Result { Ok(()) }
 
-#[cfg(unix)]
+#[cfg(any(
+	linux_android,
+	netbsdlike,
+	target_os = "aix",
+	target_os = "freebsd",
+))]
 /// Some distributions ship with very low defaults for thread counts; similar to
 /// low default file descriptor limits. But unlike fd's, thread limit is rarely
 /// reached, though on large systems (32+ cores) shipping with defaults of
@@ -47,7 +52,12 @@ pub fn maximize_thread_limit() -> Result {
 	Ok(())
 }
 
-#[cfg(not(unix))]
+#[cfg(not(any(
+	linux_android,
+	netbsdlike,
+	target_os = "aix",
+	target_os = "freebsd",
+)))]
 pub fn maximize_thread_limit() -> Result { Ok(()) }
 
 #[cfg(unix)]