refactor: stale module-level docstrings still reference `src/` path instead of `pulseengine/core/`

[Codex-Crusader]

Summary

After the v0.3 repository restructure (which moved all core logic from src/ to pulseengine/core/), the module-level docstrings in several pulseengine/core/ files still reference the old src/ path in their first line. This violates the principle of keeping documentation accurate and confuses contributors.

Affected files

File	Stale docstring first line
pulseengine/core/signals.py	src/signals.py — Signal scoring…
pulseengine/core/price.py	src/price.py — Price data fetching…
pulseengine/core/errors.py	src/errors.py — Shared exception types…

All three files have been physically moved to pulseengine/core/ but their docstrings still say src/.

Proposed fix

Update each docstring to reflect the actual canonical path:

# signals.py — before
"""
src/signals.py — Signal scoring, event detection, and news-asset correlation.
...
"""

# signals.py — after
"""
pulseengine/core/signals.py — Signal scoring, event detection, and news-asset correlation.
...
"""

Check all remaining files in pulseengine/core/ for the same issue.