Avoid eager static initialization in ORT (#31964)
## Description
Avoid two eager dynamic initializers in ONNX Runtime:
- Replace the model-package manifest parser's namespace-scope
`std::set<std::string>` key tables with constexpr
`std::array<std::string_view>` tables.
- Move the AVX2 S8-to-U8 transformer's namespace-scope
`std::unordered_map` to a function-local static so it is initialized
only if the transformer runs.
This preserves lookup behavior and source membership while removing
unnecessary process-startup initialization.
## Motivation and Context
An ARM64 binary-size regression report showed `.CRT$XCU` increasing from
176 to 192 bytes. Exact before/after PDB comparison identified two
additional initializer entries:
- `_GLOBAL__sub_I_manifest_parser.cc`
- `_GLOBAL__sub_I_avx2_weight_s8_to_u8.cc`
Each ARM64 `.CRT$XCU` entry is an 8-byte function pointer, accounting
for the full 16-byte increase. The issue was isolated to eager
initialization.
## Validation
- ARM64 official ONNX Runtime build passed.
- Benchmark-equivalent `dumpbin /headers /Coffgroup` reports `.CRT$XCU =
0xB0` (176 bytes), restoring the baseline from 192 bytes.
- Both causal initializer symbols are absent from the fixed ARM64 PDB.
- x64 official inference-engine E2E suite passed: 11/11.
- Win32k-lockdown sandbox test passed: 1/1.
- `clang-format --dry-run --Werror` passed.
- Local AI code review reported no findings.
