Add initial QEMU tests for diskann-wide.

[hildebrandmw]

This adds QEMU tests to CI to provide coverage of our architecture detection mechanisms.

QEMU is an emulator that, amongst other things, allows to run binaries as if we were running on an older CPU. This can help catch improper compilation when we target older CPUs. Unfortunately, user-space emulation of AVX-512 doesn't go the other way, so we can't quite use this to test our AVX-512 code on non-AVX-512 hosts.

Additionally, this closes #694 by changing the application of -Dwarnings from overriding RUSTFLAGS to instead passing it via the --config flag. The former effectively disables our .cargo/config.toml, which in-turn disabled the x86-64-v3 CPU target.

An example of a failing run can be seen here, where the CI test fails due to an illegal instruction found by QEMU.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.01%. Comparing base (7b87844) to head (578c706).

Additional details and impacted files

@@           Coverage Diff            @@
##             main     #719    +/-   ##
========================================
  Coverage   89.01%   89.01%            
========================================
  Files         428      428            
  Lines       78151    78294   +143     
========================================
+ Hits        69563    69695   +132     
- Misses       8588     8599    +11     

see 6 files with indirect coverage changes

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[Copilot]

Pull request overview

This PR adds QEMU-based testing to CI to validate that the architecture detection mechanisms work correctly when compiled for older x86-64 CPUs. It also fixes issue #694 by changing how compiler warnings are applied - using --config flag instead of overriding RUSTFLAGS environment variable, which was preventing .cargo/config.toml from being used.

Changes:

• Changed from RUSTFLAGS environment variable to RUST_CONFIG with --config flag to preserve .cargo/config.toml settings
• Added new qemu job that compiles tests for x86-64 baseline and runs them under QEMU emulation to catch illegal instructions
• Updated all cargo command invocations across test jobs to use --config "$RUST_CONFIG"

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