tui: add real Bus Diagnostics snapshot view (M6.5.3)

[KeyffMS]

Implements M6.5.3 from #26: replace the product Bus Diagnostics placeholder with a presentation-only projection of existing immutable diagnostics snapshots.

Scope:

• dedicated Bus Diagnostics renderer owned by lantern-tui;
• consumes only ApplicationView / MonitoringDiagnosticsView already projected by application/runtime;
• displays RTU round-trip p95 and bus utilization;
• displays timeout and QueueFull pressure counters;
• displays poll-plan utilization, deadline skips and result drops;
• displays bounded CSV/fault/diagnostics consumer drop counters;
• explicitly emits no Modbus requests and owns no transport state;
• inactive session remains fail-closed (Verified session required);
• tests assert semantic snapshot formatting and that the product Bus Diagnostics route no longer renders the historical placeholder.

No new runtime timer, bus handle, queue owner or transport state is introduced. The final diff is limited to two lantern-tui files.

Acceptance: CI #1026 / run 34626701365 completed successfully on exact head d920c67d775fc66747a313d576b4a4fd3b7ddfea; quality/coverage, architecture and supply-chain gates are green.

Refs #25
Refs #26