[v0.3] Create pulseengine/web/ surface with restricted Streamlit demo build

[Codex-Crusader]

Parent Issue

Part of #31 — v0.3 Repo Restructure

Depends on #32 — core/ must be in place before this work begins.

Context

The web demo currently runs off the same dashboard/main.py as the local app. Under the v0.3 restructure, pulseengine/web/ becomes its own dedicated surface — a deliberately stripped-down Streamlit app that imports from core/ but enforces the constraints required for Streamlit Community Cloud deployment.

The web demo's only job is to give visitors a taste of the tool and send them to the local download.

Hard Constraints for web/

These are non-negotiable — violations break the Streamlit Community Cloud deployment model:

• No local model inference — no FinBERT, no heavy ML models
• No file I/O — cannot write to disk, no snapshot persistence
• No state between requests — each session is stateless
• No backtesting — compute is too heavy for free tier
• No arbitrary ticker lookup — local-only feature

Locked Feature UX

Every feature that exists in local/ but not web/ must display a clear prompt to download the local app. The ROADMAP specifies: "Locked features in the web demo display a prompt to download the local app."

Files to Create

File	Purpose
pulseengine/web/dashboard.py	Restricted Streamlit demo — imports from core/, no heavy ops
pulseengine/web/__init__.py	Package init

Feature Matrix for web/

Feature	Include in web/?
All 24 tracked assets	Yes
Signal score + explanation	Yes
Price chart	Yes
News sentiment feed	Yes
Market heatmap	Yes
Category overview	Yes
Arbitrary ticker lookup	No — show download prompt
Backtesting	No — show download prompt
Historical snapshots	No — show download prompt
Export to CSV / PDF	No — show download prompt
FinBERT local model	No — show download prompt
Custom RSS feeds	No — show download prompt
Offline mode	No — show download prompt

Acceptance Criteria

• pulseengine/web/dashboard.py deploys on Streamlit Community Cloud without errors
• All 24 tracked assets display correctly with live signal scores
• Locked features show a clear "Download the local app" call-to-action
• No file system writes occur at any point in the web session
• No imports from local/ (only from core/)
• pulseengine/web/__init__.py present
• No user data is stored or persisted anywhere (privacy commitment per ROADMAP)

Privacy Note

From the ROADMAP: "We store nothing. Ever. The web demo is architecturally incapable of retaining user data." This constraint must be maintained in the implementation.