fix: honor pyproject-declared source roots in Python import resolution

[jkri-ch]

Problem

Projects that keep importable code in a subdirectory of the repo root (e.g. scripts/ run with PYTHONPATH=scripts, declared via [tool.pytest.ini_options] pythonpath = ["scripts"]) get completely broken import resolution:

• resolve_absolute_import only tries the scan root and the project root, so import riot_hydration never resolves to scripts/riot_hydration/__init__.py — the dependency graph reports 0 importers for every module.
• The Python test-coverage import-spec mapper (languages/python/test_coverage.py::resolve_import_spec) only tries root-relative and src/-prefixed candidates, so every from mypkg.store import ... in a test file fails to map — every production module is flagged "Untested module (N LOC, 0 importers)" even when it has a dedicated multi-thousand-line test file.

Observed on a real ~115k-LOC project (desloppify scan --path .): 205 open test_coverage findings, ~80% of which were false positives, dragging Test health to 37% — independently adjudicated as detector noise by two blind review rounds.

Fix

New desloppify/languages/python/source_roots.py with a cached declared_source_roots(project_root) that reads source roots from pyproject.toml:

• [tool.desloppify] python_source_roots (explicit override)
• [tool.pytest.ini_options] pythonpath
• [tool.mypy] mypy_path

(absolute paths, .. traversal, and . are dropped; duplicates removed)

• resolve_absolute_import now tries scan root → project root → declared roots (existing order preserved).
• resolve_import_spec extends its layout-prefix list (src/) with "<root>/" for each declared root.

Verification

• 11 new tests in desloppify/languages/python/tests/test_py_source_roots.py (declaration parsing, resolution precedence, spec mapping, src/ regression).
• Full suite: no new failures vs a clean checkout (the handful of failing tests on main — ruby phases, bash unused-imports, review-prepare dry-run — fail identically before and after this change).
• Against the original project: declared roots: ('scripts',); riot_hydration.store resolves to scripts/riot_hydration/store.py; test-spec mapping for tests/unit/riot_hydration/test_riot_hydration_store.py → scripts/riot_hydration/store.py.

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