Fix stop_server coverage gap, add return types, unify console output

[tschm]

Summary

Addresses three quality findings from a Rhiza quality assessment. All changes are confined to src/pycharting/api/interface.py and tests/test_interface.py.

Tracked in the fork at tschm#7, tschm#8, tschm#9.

Changes

7 — Test-isolation / coverage gap (interface.py:285)
The cleanup_globals fixture used global _active_server, which only rebinds the test module's imported copy and never touches interface._active_server. The running server therefore leaked across tests, leaving the no-server branch of stop_server() uncovered and making test_status_when_no_server intermittently fail. The fixture now resets the live module global (stop + set None) before and after each test; test_stop_server_when_not_running forces the no-server branch and asserts on the message. Coverage is back to 100% and deterministic across repeated runs.

8 — Return-type annotations
Added stop_server() -> None and _repr_html_() -> str, matching the rest of the public API surface.

9 — Unified console output
Routed all user-facing console output through a single _notify() helper instead of scattered bare print() calls. No bare print() remains in src/pycharting/; visible stdout behavior is preserved.

Verification

Gate	Result
make fmt	✅
make typecheck	✅ All checks passed
make docs-coverage	✅ 100%
make deptry	✅ No issues
make test	✅ 143 passed, 100.00% coverage (×3 consecutive runs)

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR improves the stability and quality of the pycharting public API interface by fixing test isolation around the module-level server global, adding missing return type annotations, and centralizing user-facing console output.

Changes:

• Fixes test isolation by resetting pycharting.api.interface._active_server (the live module global) before/after each test and adds a deterministic no-server branch test for stop_server().
• Adds return type annotations for stop_server() -> None and _repr_html_() -> str.
• Introduces _notify() and routes user-facing stdout messages through it (replacing scattered direct print() calls).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
tests/test_interface.py	Fixes global state cleanup to target the live interface module and adds deterministic coverage for the stop_server() no-op branch.
src/pycharting/api/interface.py	Adds _notify() for centralized console output and adds missing return type annotations for public/Jupyter-facing functions.

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