Merge bitcoin/bitcoin#31449: coins,refactor: Reduce getblockstats RPC UTXO overhead estimation

5f36e0ff1e rpc: fix getblockstats UTXO overhead accounting (Lőrinc)
76190489e6 coins: pack `Coin` height/coinbase consistently (Lőrinc)
1f309d1aa2 coins: make `Coin::fCoinBase` a bool (Lőrinc)

Pull request description:

  The [`getblockstats` RPC](https://github.com/bitcoin/bitcoin/pull/10757) currently overestimates UTXO overhead by treating the `fCoinBase` bitfield as an additional `bool` in `PER_UTXO_OVERHEAD`.
  However, `fCoinBase` and `nHeight` are stored as bitfields and effectively packed into a single 32-bit value; counting an extra bool in the overhead calculation is unnecessary.

  This PR introduces the following changes across three commits:
  * Store `fCoinBase` as a `bool` bitfield to reduce implicit conversions at call sites.
  * Use a consistent height/coinbase packing style across `Coin` serialization, undo serialization, and coinstats hashing (casting `nHeight` to `uint32_t` before shifting to avoid signed-promotion UB).
  * Adjust UTXO overhead estimation to match the actual `Coin` layout and update the related tests accordingly.

ACKs for top commit:
  achow101:
    ACK 5f36e0ff1e
  sedited:
    ACK 5f36e0ff1e
  vasild:
    ACK 5f36e0ff1e
  optout21:
    crACK 5f36e0ff1e

Tree-SHA512: f4a44debed358e9e130da9d6fae5f89289daa34f0bdb7155edc3e9b691c219451f4c80b1e16aca5b011f0fa2fa975484ef1af4ca4563b7c6ba4ca9dd133f30be

src/kernel/coinstats.cpp

@@ -46,7 +46,7 @@ template <typename T>
 static void TxOutSer(T& ss, const COutPoint& outpoint, const Coin& coin)
 {
     ss << outpoint;
-    ss << static_cast<uint32_t>((coin.nHeight << 1) + coin.fCoinBase);
+    ss << ((uint32_t{coin.nHeight} << 1) | uint32_t{coin.fCoinBase});
     ss << coin.out;
 }