transaction: Implement Coinbase type with explicit API design

We provide explicit convenience methods (compute_txid, compute_wtxid)
rather than implementing Deref to only expose methods that make sense
for coinbase transactions. This prevents inheritance of the entire
Transaction API.

inner() serves as an escape hatch when full Transaction access is needed.
This forces the user to be explicit about when they need the underlying
Transaction functionalities.

Following this approach, we avoid implicit inheritance through Deref.

We also added assume_* methods to make it clear that the caller is
responsible for ensuring the transaction is actually a coinbase transaction
in the first position of a block

Author: jrakibi

bitcoin/src/blockdata/transaction.rs

@@ -1169,6 +1169,42 @@ internals::transparent_newtype! {
         pub fn assume_coinbase_ref(inner: &_) -> &Self;
     }
 }
+
+impl Coinbase {
+    /// Creates a `Coinbase` wrapper assuming this transaction is a coinbase transaction.
+    ///
+    /// This method does not validate that the transaction is actually a coinbase transaction.
+    /// The caller must ensure that this transaction is indeed a valid coinbase transaction.
+    pub fn assume_coinbase(tx: Transaction) -> Self {
+        Self(tx)
+    }
+
+    /// Returns a reference to the underlying transaction.
+    ///
+    /// Warning: The coinbase input contains dummy prevouts that should not be treated as real prevouts.
+    #[doc(alias = "as_inner")]
+    pub fn as_transaction(&self) -> &Transaction { &self.0 }
+
+    /// Returns the underlying transaction.
+    ///
+    /// Warning: The coinbase input contains dummy prevouts that should not be treated as real prevouts.
+    #[doc(alias = "into_inner")]
+    pub fn into_transaction(self) -> Transaction { self.0 }
+
+    /// Computes the [`Txid`] of this coinbase transaction.
+    pub fn compute_txid(&self) -> Txid {
+        self.0.compute_txid()
+    }
+
+    /// Returns the wtxid of this coinbase transaction.
+    ///
+    /// For coinbase transactions, this is always `Wtxid::COINBASE`.
+    #[doc(alias = "compute_wtxid")]
+    pub const fn wtxid(&self) -> Wtxid {
+        Wtxid::COINBASE
+    }
+}
+
 mod sealed {
     pub trait Sealed {}
     impl Sealed for super::Transaction {}