coins: make CCoinsView methods pure virtual

`CCoinsView` provided default no-op implementations, which allowed constructing a bare view and silently getting dummy behavior.
Make all interface methods pure virtual and remove the legacy default definitions from `coins.cpp` so callers must choose an explicit implementation.
Move the virtual destructor to the beginning to avoid mixing it between the methods.
No-op backing behavior remains available via `CoinsViewEmpty`.

src/test/fuzz/coinscache_sim.cpp

@@ -247,7 +247,7 @@ FUZZ_TARGET(coinscache_sim)
         CallOneOf(
             provider,
 
-            [&]() { // GetCoin
+            [&]() { // PeekCoin/GetCoin
                 uint32_t outpointidx = provider.ConsumeIntegralInRange<uint32_t>(0, NUM_OUTPOINTS - 1);
                 // Look up in simulation data.
                 auto sim = lookup(outpointidx);