chore(demo): add live span/log waterfall page

[overbalance]

What problem is this solving?

Groundwork for EMBR-12823 (span links + related-logs attribute on the soft-nav performance span): before designing that, we need to see how soft-nav spans and logs overlap in time. This adds a developer-facing demo page that visualizes the SDK's live span/log stream as a waterfall.

Short description of the changes

• New Waterfall demo page (demo/frontend/waterfall/) with a nav link after Playground. Soft-nav buttons (Home/A/B pushState), Info/Warn/Error log buttons, and the visualization.
• Live capture via in-memory SpanExporter/LogRecordExporter added to the SDK's exporter chain (setupSDK gained optional extra-exporter params).
• Waterfall viz: spans as bars colored by emb.type (session parts get the brand green), logs as severity-colored circles on a shared timeline; hide-network toggle; click a span to zoom to a fixed 1.5s window with horizontal scroll and ms ticks; cursor-following tooltips (edge-safe, with start/end/duration).
• Sub-app deep-link fallback: a Vite dev/preview middleware that serves the right entry for hard loads like /waterfall/a, replacing the older root-index.html redirect script (which didn't cover the new page and dropped the sub-route).

All changes are demo-only (demo/frontend/); the published SDK is untouched.

How was this tested?

• Driven manually in the dev server (:4847): navigate + emit logs, verified live rendering, per-type colors, click-to-zoom + horizontal scroll + reset, edge-safe tooltips.
• Hard-nav verified for /waterfall/a, /soft/a, and a router demo deep link.
• npm run lint + tsc pass via the pre-commit hook.
• Ran the PR-review agent suite; addressed the correctness finding (unbounded Math.min(...) spread), exporter callback ordering, and a stale comment.

Checklist

• Demo-only change; no SDK/public API impact
• Lint + typecheck pass
• Manually verified in the browser
• Golden/integration tests — N/A (no SDK behavior change)

[github-actions]

Chrome DevTools Protocol Tracing (Script: 93.09ms, Heap: 13.21MB)

	Number of Requests	Size of Requests	Script Duration	Task Duration	Heap Used Size
Requests	+12 requests	+41.41 KB
Page Loaded			+16.97 ms	+10.03 ms	+1.17 MB
Generate 100 fetch requests			+14.19 ms	+60.57 ms	+1.49 MB
Generate 100 XHR requests			+32.37 ms	+86.71 ms	+2.75 MB
Click 100 buttons and generate 100 logs			+20.48 ms	+45.02 ms	+2.38 MB
Throw a 100 exceptions			+0.50 ms	+25.68 ms	+2.20 MB
End Session			+8.59 ms	+20.11 ms	+3.22 MB
Total	+12 requests	+41.41 KB	+93.09 ms	+248.12 ms	+13.21 MB

Lighthouse (Script Eval: 113.50ms)

	Difference	Description
Total Blocking Time	0 ms	Difference in Total Blocking Time: Sum of all time periods between FCP and Time to Interactive, when task length exceeded 50ms, expressed in milliseconds. Learn more about the Total Blocking Time metric.
Main Thread Time	+90.44 ms	Difference in Main Thread Time: Consider reducing the time spent parsing, compiling and executing JS. You may find delivering smaller JS payloads helps with this. Learn how to minimize main-thread work
Script Evaluation Time	+113.50 ms	Difference in Script Evaluation Time: Consider reducing the time spent parsing, compiling, and executing JS. You may find delivering smaller JS payloads helps with this. Learn how to reduce Javascript execution time.

Platform Tests (vite-7 es2015 gzip: 69.66KB)

vite-6 Platform Tests

	Total Uncompressed Size	Total Gzip Size
vite-6 - es2015	+203.00 KB	+69.70 KB

vite-7 Platform Tests

	Total Uncompressed Size	Total Gzip Size
vite-7 - es2015	+203.00 KB	+69.66 KB

webpack-5 Platform Tests

	Total Uncompressed Size	Total Gzip Size
webpack-5 - es2015	0 KB	0 KB

[joseph-embrace]

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