Sliding window pipeline + credit request bump + SmbClient accessors

Sliding window: each received response immediately triggers sending
the next chunk, keeping the TCP pipe full at all times. Replaces the
batch "send N, wait for all N" pattern that left gaps between windows.

Credit request bumped from 32 to 256 per request. The server grants
what it can afford — we just ask for more. Credits grow rapidly
(511 → 15,842 after 20 operations in diagnostic test).

Read pipeline: 64 KB chunks (CreditCharge=1, max concurrency).
Write pipeline: MaxWriteSize chunks (minimize overhead).
Window capped at 32 in-flight requests.

Added SmbClient::credits() and estimated_rtt() accessors.
Added diagnostic integration test for rapid pipelined writes.

521 unit tests + 15 integration tests, zero clippy warnings.

Author: vdavid

src/client/connection.rs

@@ -324,7 +324,7 @@ impl Connection {
     ) -> Result<(MessageId, Vec<u8>)> {
         let mut header = Header::new_request(command);
         header.message_id = MessageId(self.next_message_id);
-        header.credits = 32; // Request more credits.
+        header.credits = 256; // Request more credits.
         header.credit_charge = CreditCharge(1);
         header.session_id = self.session_id;
         if let Some(tid) = tree_id {
@@ -369,7 +369,7 @@ impl Connection {
     ) -> Result<(MessageId, Vec<u8>)> {
         let mut header = Header::new_request(command);
         header.message_id = MessageId(self.next_message_id);
-        header.credits = 32; // Request more credits.
+        header.credits = 256; // Request more credits.
         header.credit_charge = CreditCharge(credit_charge);
         header.session_id = self.session_id;
         if let Some(tid) = tree_id {

src/client/mod.rs

@@ -181,6 +181,16 @@ impl SmbClient {
         &self.config
     }
 
+    /// Current number of available credits.
+    pub fn credits(&self) -> u16 {
+        self.conn.credits()
+    }
+
+    /// Estimated round-trip time from the negotiate exchange.
+    pub fn estimated_rtt(&self) -> Option<Duration> {
+        self.conn.estimated_rtt()
+    }
+
     /// Get a mutable reference to the underlying connection.
     ///
     /// Needed when using [`Tree`] methods directly, since they require