Fix throughput chart not updating live (per-snapshot delta, not wall-clock rate)

[StrangeNoob]

Problem

The dashboard's Throughput chart appeared frozen — it only looked correct right after a page reload. Reported as "Throughput is not increasing. I need to refresh the page for this."

Root cause

The client throughput series computed a wall-clock rate: Δprocessed_total / Δ(Date.now arrival). The server pushes one SSE snapshot per ~1s, but arrivals are not evenly spaced — a backgrounded tab or a reconnect flushes several queued snapshots within milliseconds of each other. Dividing a normal delta by that near-zero interval produced enormous spikes, and because the sparkline auto-scales to its max, one spike crushed every real value to a flat line near the axis. The depth chart was immune because it plots an instantaneous value with no time term.

Verified server-side correctness separately (curl + a streaming-fetch probe showed processed_total rising at a clean ~1s cadence), so this is purely a client-side derivation bug.

Fix

Since the server already emits ~1 snapshot/second, the per-snapshot delta of the cumulative processed_total counter is the per-second throughput — with no timing term to destabilize it.

• web/src/lib/series.ts: replace ratePerSecond (and its Sample arrival-time struct) with processedDelta(prev, cur). A counter reset/flush (cur < prev) yields 0, never negative.
• web/src/App.tsx: track prevProcessed as a plain number ref; push processedDelta into the throughput window on each snapshot.
• web/src/lib/series.test.ts: tests for the delta, the idle-tick zero, the reset clamp, and explicitly that bunched arrivals report the true delta (no spike).
• web/dist: rebuilt embedded bundle.

Testing

• npm run test — 17/17 pass
• npm run typecheck — clean
• npm run build — dist rebuilt and committed

[StrangeNoob]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

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