8783cc8056 refactor: inline `CCoinsViewBacked` implementation (Lőrinc)
86296f276d coins: make `CCoinsView` methods pure virtual (Lőrinc)
b637566c8d coins: add explicit `CoinsViewEmpty` noop backend (Lőrinc)
90c635c01c fuzz: keep backend assertions aligned to active backend (Lőrinc)
a9f92e3497 refactor: normalize CCoinsView whitespace and signatures (Lőrinc)
38a99f3344 scripted-diff: normalize `CCoinsView` naming (Lőrinc)
06172ef0d5 refactor: rename `hashBlock` to `m_block_hash` to avoid shadowing (Lőrinc)
Pull request description:
### Problem
`CCoinsView` is the coins view interface, but historically it also provided built-in no-op behavior:
* It provided default no-op implementations (returning `std::nullopt`, `uint256()`, `false`, or `nullptr`) instead of being pure virtual.
* Callers could instantiate a bare `CCoinsView` and get silent no-op behavior.
* Mixing the interface definition with a built-in dummy implementation blurred the abstraction boundary.
### Context
This is part of the ongoing coins caching cleanup in #34280.
### Fix
This PR separates the interface from no-op behavior and makes `CCoinsView` pure virtual in incremental steps:
* Add `CoinsViewEmpty` as an explicit no-op coins view for tests, benchmarks, and temporary backends.
* Replace direct bare-`CCoinsView` test and dummy instantiations with `CoinsViewEmpty`.
* Make all `CCoinsView` methods pure virtual (`PeekCoin`, `GetCoin`, `HaveCoin`, `GetBestBlock`, `GetHeadBlocks`, `BatchWrite`, `Cursor`, `EstimateSize`).
* Remove the legacy default implementations from `coins.cpp`.
* Update fuzz and dummy backends to use `CoinsViewEmpty` explicitly.
ACKs for top commit:
w0xlt:
reACK 8783cc8056
ryanofsky:
Code review ACK 8783cc8056. Just updated comments and variable name since last review. The fuzz test code is much clearer now IMO
andrewtoth-exo:
ACK 8783cc8056
ajtowns:
ACK 8783cc8056
Tree-SHA512: cfc831578aa309788c1b5dafbfecca3de388cc4215534c3f3df24f90d7770ed37b1fd7aa134df91d611d0a1ca75929accb98d5ed7df7b52851c259e04f08e4a3
976985eccd