afb1bc120ecce2bf663093e15c93f5592c0d4a98 validation: Use dirty entry count in flush warnings and disk space checks (Pieter Wuille)
b413491a1cdd9a51f2aa10b775650f54f6785e3e coins: Keep track of number of dirty entries in `CCoinsViewCache` (Pieter Wuille)
7e52b1b945c4137e0fb05715090635ce82ed04b3 fuzz: call `EmplaceCoinInternalDANGER` as well in `SimulationTest` (Lőrinc)
Pull request description:
### Problem
Now that non-wiping flushes are possible (#28280, #28233), the cache may be mostly clean at flush time.
But the flush warning, disk-space check, and benchmark logging still used total cache size, so a node with a 10 GiB cache that only needs to write a small fraction of dirty entries could still trigger a scary warning via the disk-space checks.
The previous `DynamicMemoryUsage` metric was also fundamentally wrong for estimating disk writes, even before non-wiping flushes. In-memory coin size differs from on-disk write size due to LevelDB overhead, log doubling, and compaction.
The warning also only fired in `FlushStateToDisk`, so `AssumeUTXO` snapshot loads never warned at all.
### Fix
This PR tracks the actual number of dirty entries via `m_dirty_count` in `CCoinsViewCache`, maintained alongside the existing dirty-flag linked list, `SanityCheck` cross-validating both counts.
The warning and benchmark log move from `FlushStateToDisk` down to `CCoinsViewDB::BatchWrite`, where the actual I/O happens. This is the single place all flush paths converge (regular flushes, syncs, and snapshot loads), so the warning now fires correctly for `AssumeUTXO` too.
The threshold changes from 1 GiB of memory to 10 million dirty entries, which is roughly equivalent but avoids the in-memory vs on-disk size confusion.
The disk-space safety check now uses `GetDirtyCount()` with the existing conservative 48-byte-per-entry estimate, preventing unnecessary shutdowns when the cache is large but mostly clean.
---
Note: the first commit adds fuzz coverage for `EmplaceCoinInternalDANGER` in `SimulationTest` to exercise the accounting paths before modifying them.
Note: this is a revival of #31703 with all outstanding review feedback addressed.
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