perf: move sampling to a background thread and cache static driver facts

[crazyguitar]

Purpose

Cold start blocked on two full sampling passes plus NVML/amdsmi driver init before the first frame, Nvml::init()/nvmlShutdown ran every tick, and any slow driver call froze rendering and input.

• cargo fmt --check passes
• cargo clippy -- -D warnings passes
• cargo test passes
• Tested on RDMA-capable hardware (or explained why not in the Test Plan)
• Docs/README updated if behavior or flags changed

[Copilot]

Pull request overview

This PR moves hardware/stat sampling off the UI thread into a dedicated background sampler and caches static GPU-driver facts, reducing cold-start latency and preventing slow driver calls from freezing rendering/input.

Changes:

• Add a background Sampler thread that collects subsystem snapshots into a single-slot mailbox for the UI.
• Refactor TUI state updates to consume sampler snapshots with per-subsystem baselines and staleness/death reporting.
• Cache “static while driver is loaded” GPU metadata (NVML + XGMI topology) to avoid repeated expensive queries.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/sampler.rs	New background sampling thread + mailbox (Snapshot) API.
src/tui/app.rs	Replace synchronous polling with apply_snapshot, add per-subsystem baselines, and add tests.
src/main.rs	Wire the sampler into the TUI main loop and propagate refresh interval changes.
src/tui/ui.rs	Add “initializing/failed” placeholders and status-bar warnings for stalled/dead sampler.
src/nvlink.rs	Cache NVML init for process lifetime and cache per-link static metadata.
src/xgmi.rs	Cache XGMI peer adjacency matrix and reuse it during per-GPU snapshot construction.
src/trace.rs	Timestamp trace samples using sampler-side timestamps.
src/stat.rs	Document intentional non-caching of RDMA port link rate due to retraining/down-port behavior.

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[Copilot]

Pull request overview

Copilot reviewed 8 out of 8 changed files in this pull request and generated 4 comments.

[Copilot]

Pull request overview

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.