S人ving Privàte Ryän

Fixes problems with non-ASCII characters in paths. Fixes #3424.

Changes
- Bump the minimum supported Clang version to 11 to support std::filesystem
- Use std::optional for empty values instead of string sentinels like "None"
- Remove the FixedString class
- Move the EvalFile network path from the network struct to the engine
- Simplify parts of the command-line handling
- Remove the old dual-net logic from export_net
- And ofc we now support non-ascii paths for windows

Behavior Change
- Because the EvalFile is no longer part of the network, it won't be included in the hashing, thus the network sharing is no longer dependent on the EvalFile path
- As a result, if the binary version and network are the same, changing only the EvalFile location will now reuse the same shared network

This also fixes a master bug where:

```
export_net None
setoption name EvalFile value None
export_net None
Failed to export a net
```

would fail on the second export. With this patch, the second export succeeds.

Quick Fishtest Check that nets load like normal.
https://tests.stockfishchess.org/tests/view/6a455593f97ff95f787954da

<img width="833" height="1137" alt="image" src="https://github.com/user-attachments/assets/fffee63b-c63a-4d5e-b13d-87d58bc28c88" />

closes https://github.com/official-stockfish/Stockfish/pull/6937

No functional change

Co-authored-by: anematode <timothy.herchen@gmail.com>

.github/workflows/avx2_compilers.yml

@@ -22,7 +22,6 @@ jobs:
           - { name: gcc-15, comp: gcc, cxx: g++, image: "gcc:15" }
 
           # Using silkeh/clang for older versions
-          - { name: clang-10, comp: clang, cxx: clang++, image: "silkeh/clang:10", is_clang: true, ver: "10" }
           - { name: clang-11, comp: clang, cxx: clang++, image: "silkeh/clang:11", is_clang: true, ver: "11" }
           - { name: clang-12, comp: clang, cxx: clang++, image: "silkeh/clang:12", is_clang: true, ver: "12" }
           - { name: clang-13, comp: clang, cxx: clang++, image: "silkeh/clang:13", is_clang: true, ver: "13" }