docs: rewrite contribution issues to reflect current implementation

[TheZupZup]

Motivation

• The project README mixed implemented features with aspirational items, making contribution triage unclear.
• Maintainable, immediate tasks are needed so contributors can start work based on what the code actually provides today.

Description

• Add docs/ISSUES_CURRENT_STATE.md which maps the codebase into "Fully implemented", "Partially implemented / unstable", and "Missing" areas and lists 8 concrete, beginner-friendly issues.
• Each issue is written with Overview, What needs to be done, Goal, and Notes and includes suggested labels like good first issue or help wanted so tasks are immediately startable.
• This is a documentation-only change that does not modify runtime code or behavior (new file: docs/ISSUES_CURRENT_STATE.md).

Testing

• Ran the full test suite with pytest -q which succeeded: 64 passed.
• Because this is docs-only, no functional tests or runtime changes were required.

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Codex Task