qwen3: docs/dead-code cleanup — stale records, deleted-tool references, unused consts #248

Background

docs/models/qwen3/ accumulated bring-up records that now describe deleted tooling or completed work, and the crate carries a few verified-dead items. docs/models/qwen3/roadmap.md lists the per-item verdicts.

Current state

• model-crate.md advertises a deleted bench target; it and kernels-crate.md use an obsolete directory layout in every command.
• tp-design.md describes the implemented TP runtime as future work.
• kv-pressure-hang.md holds lasting lessons (full-lifetime KV reservation) that belong in docs/lessons/.
• Dead code: two pub consts in batch_decode_trace.rs with zero readers; probe_model()/ModelInfo has no caller (the server inlines its own detection — the same dead pair exists in the qwen35 crate).

Desired outcome

• Docs describe the crate that exists: commands runnable, TL;DRs true, lessons lifted before deletion, index.md rows updated.
• Dead items removed (or probe_model actually wired into the server, if that's the better call).

Acceptance boundary

• Per repo convention: every command left in a doc has been run before committing.
• No behavior change; clippy and the test suite stay green.