Summary

Adds a dedicated Topology tab (T) that visualises intra-node GPU
interconnect structure — NvLink connections (GPU↔GPU, GPU↔NvSwitch),
NUMA affinity, and PCIe lanes. Includes both a graph-style ASCII layout
and a nvidia-smi topo -m-equivalent matrix fallback. Works on systems
without NvLink by dropping to a PCIe-only rendering.

Implementation

• New module: src/ui/topology/ — pure-logic core split across
  five files, each under the 500-line soft limit:
  • mod.rs — TopologyModel + TopologyViewMode assembled from
    GpuInfo.nvlink_remote_devices + numa_node_id + detail.
  • classify_edge.rs — NVn bandwidth-hint classifier with a
    dominant-generation picker; falls back to generic NV when
    bandwidth is unknown.
  • layout.rs — NUMA-aware grid layout; picks horizontal vs
    vertical box stacking based on terminal width.
  • graph_render.rs — ASCII NUMA boxes with GPUs and edges.
  • matrix_render.rs — nvidia-smi topo -m table, fits the column
    widths to the GPU count.
• Orchestrator: src/ui/renderers/topology_renderer.rs — draws
  the panel for the selected host; falls back to matrix on terminals
  narrower than 100 columns so the content never overflows on
  80-column sessions.
• Event routing: T jumps to the tab; M toggles graph/matrix
  while the tab is active. Mode precedence ladder updated:
  filter-edit > replay-timecode > Users-tab keys > Topology-tab keys >
  global > replay.
• Data model: extends NvLinkRemoteDevice with
  bandwidth_mb_s: Option<u32> so the NVn generation classifier can
  derive labels like NV5 from the hint. Existing construction sites
  (NVIDIA reader, mock templates, test fixtures) updated.
• Prometheus round-trip: adds bandwidth_mb_s as an optional
  label on all_smi_nvlink_remote_device_type. Parser accepts the
  label when present, rejects absurd upstream values, and remains
  backward-compatible with pre-feat(tui): topology view tab ('T') — NvLink/NUMA/PCIe graph and matrix #190 exporters that omit it.
• Mock template: ALL_SMI_MOCK_TOPOLOGY=1 emits a DGX-style
  8-GPU, 2-NUMA, 64-link (7 GPU + 1 switch per GPU) topology so the
  tab can be exercised without real hardware.
• Help overlay + README updated to document T, M, and
  graceful-degradation behaviour.

Graceful degradation

• No NvLink present → graph shows NUMA boxes with PCIe-only GPUs.
• Non-NVIDIA hosts → NUMA + PCIe groupings only; no SYS/NVn
  vocabulary.
• nvlink_remote_devices empty → dim "no active NvLinks" placeholder.
• No NUMA topology → single synthetic NUMA ? box.
• Terminals < 100 columns → automatic matrix fallback with a hint.

Testing

• Unit tests (library + binaries): layout with 2/4/8 GPUs on 1 and 2
  NUMAs; edge classification covering full mesh / switch mesh / no
  NvLink; matrix formatting (header + legend + cell sizing); graph
  rendering (horizontal and vertical stacking); view mode toggle;
  NvLink bandwidth round-trip + backward-compat with old exporters.
• Integration: hardware_details_integration_test confirms the
  Prometheus exporter + network parser handle the new label without
  breaking older scrapes.
• Mock template unit tests (bypass env-var to avoid test-thread
  races): NUMA split, 64-link count, instance labelling, empty-input
  no-op.
• cargo test --lib (817 pass), cargo test --bin all-smi (932
  pass), cargo test --bin all-smi-mock-server --features=mock (52
  pass), cargo clippy --all-targets -- -D warnings, and
  cargo fmt --all -- --check all succeed.

Test plan

• Verify the Topology tab is reachable via T in remote mode
  against a real or mocked cluster.
• Confirm M toggles between graph and matrix modes without
  data loss.
• On a terminal resized below 100 columns, confirm the graph
  mode shows the "matrix fallback" hint and switches to matrix
  rendering.
• With ALL_SMI_MOCK_TOPOLOGY=1, confirm the mock server emits
  the DGX-style topology and the TUI renders it correctly.
• Scrape a pre-feat(tui): topology view tab ('T') — NvLink/NUMA/PCIe graph and matrix #190 exporter (no bandwidth_mb_s label) from
  the new TUI and confirm NvLink rows still appear in the matrix.

Closes #190