fix: correct drifted citations at commit f53b654

- traits.md: iterator.rs anchor L76→L42 (pub const trait Iterator)
- traits.md: deref.rs anchor L60→L139 (pub const trait Deref)
- error-handling.md: fix variant names CliError::Io→IoError, Parse→ParseError
  to match actual stdlib doc example at convert/mod.rs:557
- concurrency.md: mutex.rs and rwlock.rs moved to sync/poison/ subtree
  (3 link updates: mutex×2, rwlock×1)

patterns/error-handling.md

@@ -266,12 +266,12 @@ impl std::error::Error for ConnectError {
 // library/core/src/convert/mod.rs:557
 impl From<io::Error> for CliError {
     fn from(error: io::Error) -> Self {
-        CliError::Io(error)
+        CliError::IoError(error)
     }
 }
 impl From<num::ParseIntError> for CliError {
     fn from(error: num::ParseIntError) -> Self {
-        CliError::Parse(error)
+        CliError::ParseError(error)
     }
 }
 ```