Live speculative execution dashboard

[armaan-25]

What changed

• replaces the localhost dashboard with the live real-queue and speculative shadow-queue timeline
• distinguishes thinking, editing, tool, testing, served, and evicted phases with aligned SVG geometry
• adds Codex session selection, latency/cache/speculation charts, trace export, event ledger, and completed-session postmortems
• reports cache hits, speculation launches, speculation serves, and latency saved for the selected session and in total
• adds the two local Geist font assets and updates the dashboard demo event fixture
• adds speculator/docs/dashboard-integration.md for durable session IDs, authoritative history, SSE resume, lifecycle events, and product wiring

Why

Spex needs a readable live view of what Codex is doing versus what the speculator is doing in parallel. Hits visibly merge into the authoritative timeline, discarded work terminates cleanly, and secondary telemetry stays behind disclosure controls.

Scope

This PR contains only the production dashboard, its required local assets/demo fixture, and dashboard-specific documentation. Predictor, benchmark, blog, and reference-package changes are excluded.

Validation

• npm test — 51 tests passed
• node --check src/dashboard.mjs
• inline dashboard script parsed with node:vm
• git diff --check

Known follow-up

• automated browser coverage is not included
• browser history currently uses localStorage
• cache epoch reset events must be separated from true session completion when wiring durable product sessions