Fix K3 R3 router discovery with current MLite - #21

Summary:

• Keep K3 decoder-layer selection for R3 router replay in the external K3 protocol.
• Remove the dependency on the shared selector that is no longer exported by current MLite.
• Update the evidence-ID fixture to the current fingerprint schema.

Validation:

• K3 d59b7eb against current MLite main 85eacfbc: targeted R3/QAT contracts 12 passed, 1 skipped. The skip is the optional distributed-model import because Transformer Engine is unavailable in the CPU environment; the routed-expert-only QAT test passed without skipping.
• Full K3 unit suite against the same MLite main: 141 passed, 2 skipped. The additional skip requires the optional official checkpoint index fixture.
• Ruff check, Ruff format check, and git diff --check passed for the two changed files.