Refactor example apps: FastAPI gallery + pure reflex-xy showcase

[masenf]

Replace the old iframe-based examples/reflex app (which committed ~25 static chart HTML files) and remove python/reflex-xy/examples/demo_app.

• examples/fastapi: a new FastAPI app that generates chart HTML with chart.to_html() and serves a 100M-point drilldown through a POST callback route, showing xy from a non-Reflex stack.
• examples/reflex: rewritten as a pure reflex-xy showcase of the ways to link chart data (figure vars, static Chart tier, inline() token) plus reflex-native features: charts driven by state vars, hover/click/select event handlers, streaming via reflex_xy.append, and data recomputed from the on_view_change event.
• Both apps show each example's source via inspect.getsource in on-page code panels, and each has its own pyproject.toml so it runs with uv run.
• Retarget the lifecycle and visual browser smokes to run examples/fastapi with uv run and drive Chromium at its routes.
• Update packaging (sdist include, verify_sdist), the coupled tests, specs, and docs to match. No static chart HTML is committed or packaged.

[codspeed-hq]

Merging this PR will not alter performance

✅ 94 untouched benchmarks
⏩ 1 skipped benchmark¹

---

_{Comparing claude/session-chyc11 (4b6742d) with main (79b0a3f)}

Footnotes

1. 1 benchmark was skipped, so the baseline result was used instead. If it was deleted from the codebase, click here and archive it to remove it from the performance reports. ↩

[greptile-apps]

Greptile Summary

This PR replaces the old iframe-based examples with FastAPI and Reflex showcases. The main changes are:

• Adds a FastAPI chart gallery with live drilldown callbacks.
• Rewrites the Reflex app around native reflex-xy data and event flows.
• Removes committed chart HTML and the old demo app.
• Updates browser smokes, packaging checks, tests, specs, and docs.

Confidence Score: 5/5

This looks safe to merge.

Concurrent requests for one chart share a per-chart build lock. The lifecycle smoke checks exact iframe routes and waits for each frame to paint. Invalid point-count settings no longer abort app import. No blocking issues were found in the updated fixes.

T-Rex Logs

What T-Rex did

• Validated that the before/after recordings use the same gallery → drilldown → zoom workflow.
• Captured current-code evidence showing HTTP 200 responses for the gallery, drilldown, and three callback requests, and confirmed the final zoom window renders at 1100×430.
• Observed a console message about a Chromium blocking the data: favicon redirect, and confirmed it did not affect tested surfaces or indicate changed behavior.

_{Ran code and verified through T-Rex}

Important Files Changed

Filename	Overview
examples/fastapi/app.py	Adds FastAPI gallery routes and serializes concurrent first builds with per-chart locks.
examples/fastapi/live_drilldown.py	Moves the live drilldown into the FastAPI example and falls back for malformed or non-positive point counts.
scripts/reflex_lifecycle_smoke.py	Retargets the lifecycle smoke and checks every expected gallery frame individually.

_{Reviews (4): Last reviewed commit: "Address FastAPI example review feedback" | Re-trigger Greptile}

[masenf]

bringing this before it gets conflicted out again and so subsequent changes no longer have to touch the mountain of HTML files