coins: warn on oversized -dbcache

Oversized allocations can cause out-of-memory errors or [heavy swapping](https://github.com/getumbrel/umbrel-os/issues/64#issuecomment-663637321), [grinding the system to a halt](https://x.com/murchandamus/status/1964432335849607224).

`LogOversizedDbCache()` now emits a startup warning if the configured `-dbcache` exceeds a cap derived from system RAM, using the same parsing/clamping as cache sizing via CalculateDbCacheBytes(). This isn't meant as a recommended setting, rather a likely upper limit.

Note that we're not modifying the set value, just issuing a warning.
Also note that the 75% calculation is rounded for the last two numbers since we have to divide first before multiplying, otherwise we wouldn't stay inside size_t on 32-bit systems - and this was simpler than casting back and forth.

We could have chosen the remaining free memory for the warning (e.g. warn if free memory is less than 1 GiB), but this is just a heuristic, we assumed that on systems with a lot of memory, other processes are also running, while memory constrained ones run only Core.

If total RAM < 2 GiB, cap is `DEFAULT_DB_CACHE` (`450 MiB`), otherwise it's 75% of total RAM.
The threshold is chosen to be close to values commonly used in [raspiblitz](https://github.com/raspiblitz/raspiblitz/blob/dev/home.admin/_provision.setup.sh#L98-L115) for common setups:

| Total RAM | `dbcache` (MiB) | raspiblitz % | proposed cap (MiB) |
|----------:|----------------:|-------------:|-------------------:|
|     1 GiB |             512 |        50.0% |               450* |
|     2 GiB |            1536 |        75.0% |               1536 |
|     4 GiB |            2560 |        62.5% |               3072 |
|     8 GiB |            4096 |        50.0% |               6144 |
|    16 GiB |            4096 |        25.0% |              12288 |
|    32 GiB |            4096 |        12.5% |              24576 |

[Umbrel issues](https://github.com/getumbrel/umbrel-os/issues/64#issuecomment-663816367) also mention 75% being the upper limit.

Starting `bitcoind` on an 8 GiB rpi4b with a dbcache of 7 GiB:
> ./build/bin/bitcoind -dbcache=7000

warns now as follows:
```
2025-09-07T17:24:29Z [warning] A 7000 MiB dbcache may be too large for a system memory of only 7800 MiB.
2025-09-07T17:24:29Z Cache configuration:
2025-09-07T17:24:29Z * Using 2.0 MiB for block index database
2025-09-07T17:24:29Z * Using 8.0 MiB for chain state database
2025-09-07T17:24:29Z * Using 6990.0 MiB for in-memory UTXO set (plus up to 286.1 MiB of unused mempool space)
```

Besides the [godbolt](https://godbolt.org/z/EPsaE3xTj) reproducers for the new total memory method, we also tested the warnings manually on:
- [x] Apple M4 Max, macOS 15.6.1
- [x] Intel Core i9-9900K, Ubuntu 24.04.2 LTS
- [x] Raspberry Pi 4 Model B, Armbian Linux 6.12.22-current-bcm2711
- [x] Intel Xeon x64, Windows 11 Home Version 24H2, OS Build 26100.4351

Co-authored-by: stickies-v <stickies-v@protonmail.com>
Co-authored-by: Hodlinator <172445034+hodlinator@users.noreply.github.com>
Co-authored-by: w0xlt <woltx@protonmail.com>

src/init.cpp

@@ -1767,6 +1767,7 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
     // ********************************************************* Step 7: load block chain
 
     // cache size calculations
+    node::LogOversizedDbCache(args);
     const auto [index_cache_sizes, kernel_cache_sizes] = CalculateCacheSizes(args, g_enabled_filter_types.size());
 
     LogInfo("Cache configuration:");