seaweed-volume: add CRC32-Castagnoli checksum module

Uses crc32c crate matching Go's crc32.MakeTable(crc32.Castagnoli).
Includes legacy Value() function for backward compat with pre-3.09 volumes.
4 unit tests.

seaweed-volume/src/storage/needle/crc.rs

@@ -0,0 +1,73 @@
+//! CRC32-Castagnoli checksum for needle data integrity.
+//!
+//! Matches Go's `crc32.MakeTable(crc32.Castagnoli)` exactly.
+//! The CRC is stored as raw u32 (not the `.Value()` legacy transform).
+
+/// CRC32-Castagnoli checksum wrapper.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
+pub struct CRC(pub u32);
+
+impl CRC {
+    /// Compute CRC from a byte slice (starting from 0).
+    pub fn new(data: &[u8]) -> Self {
+        CRC(0).update(data)
+    }
+
+    /// Update the CRC with additional bytes.
+    pub fn update(self, data: &[u8]) -> Self {
+        CRC(crc32c::crc32c_append(self.0, data))
+    }
+
+    /// Legacy `.Value()` function — deprecated in Go but needed for backward compat check.
+    /// Formula: (crc >> 15 | crc << 17) + 0xa282ead8
+    pub fn legacy_value(&self) -> u32 {
+        (self.0 >> 15 | self.0 << 17).wrapping_add(0xa282ead8)
+    }
+}
+
+impl From<u32> for CRC {
+    fn from(v: u32) -> Self {
+        CRC(v)
+    }
+}
+
+impl From<CRC> for u32 {
+    fn from(c: CRC) -> Self {
+        c.0
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_crc_empty() {
+        let crc = CRC::new(&[]);
+        assert_eq!(crc.0, 0);
+    }
+
+    #[test]
+    fn test_crc_known_value() {
+        // CRC32-C of "hello" — verify it produces a non-zero deterministic value
+        let crc = CRC::new(b"hello");
+        assert_ne!(crc.0, 0);
+        // Same input produces same output
+        assert_eq!(crc, CRC::new(b"hello"));
+    }
+
+    #[test]
+    fn test_crc_incremental() {
+        let crc1 = CRC::new(b"hello world");
+        let crc2 = CRC::new(b"hello").update(b" world");
+        assert_eq!(crc1, crc2);
+    }
+
+    #[test]
+    fn test_crc_legacy_value() {
+        let crc = CRC(0x12345678);
+        let v = crc.legacy_value();
+        let expected = (0x12345678u32 >> 15 | 0x12345678u32 << 17).wrapping_add(0xa282ead8);
+        assert_eq!(v, expected);
+    }
+}