Tenstorrent host-free decode graph: capture, replay, and on-device state advance

Row: BACKEND-TENSTORRENT-HOST-FREE-FORWARD

Summary

The Tenstorrent Blackhole decode path cannot run a captured mesh-trace while any to_vector / host write sits inside the captured region. The trace-runner spike measured that flipping the T=1 hybrid thresholds is not enough: capture aborts on Reads are not supported during trace capture.

This issue owns:

• BACKEND-TENSTORRENT-TRACE-RUNNER (SPIKE, decision record)
• BACKEND-TENSTORRENT-HOST-FREE-FORWARD (ACTIVE: R1-R3b plus R2 on-device cur_pos advance)

What landed on PR #1053

Env-gated VT_TT_HOST_FREE_DECODE decode-graph capture. On a Blackhole P150, Qwen3-0.6B "Hello" at 80 tokens completed 79 replays with no hang, 5.8x vs eager, 22/22 argmax vs the per-step-copy baseline. Default path is inert.

Still owed

• Fresh review of the host-free decode graph
• Operator rerun of the 80-token no-hang gate and the TT golden on card
• test_qwen3_paged_engine still times out under the flag

Specs

• .agents/specs/tenstorrent-host-free-forward.md
• .agents/specs/tenstorrent-host-free-r1.md
• .agents/specs/tenstorrent-host-free-r2.md
• .agents/specs/tenstorrent-trace-runner.md

Start with .agents/specs/tenstorrent-host-free-forward.md, the R1/R2 specs, and PR #1053; inspect the captured decode graph and the test_qwen3_paged_engine timeout. Done means completing the fresh review, rerunning the 80-token no-hang gate and TT golden on card, and documenting the timeout outcome.