Add Hypothesis property tests for 6 previously uncovered modules

[Copilot]

Issue #1430 identified gaps in property-based test coverage. Six py3plex modules had no corresponding tests/property/ files.

New test files (124 tests total)

File	Tests	Coverage
test_diagnostics_properties.py	15	DiagnosticSeverity, FixSuggestion, Diagnostic construction/serialisation
test_optimizer_properties.py	36	Logical/physical plan node dataclasses (LogicalFilter, LogicalScan, LogicalProject, etc.)
test_out_of_core_properties.py	19	_parse_condition_kwargs, OutOfCoreNetwork construction and info() schema
test_experiments_properties.py	15	Experiment construction, compute_id() determinism, identity/round-trip invariants
test_linter_properties.py	25	LintIssue severity constants, LINT_ERROR_CODES structure, format invariants
test_ergonomics_properties.py	15	quick_network, quick_analysis, show_network_summary

Notable API quirks surfaced during test authorship

• net.get_layers() returns a 3-tuple (layer_names, graph_objects, edge_dict) — not a flat iterable of names. Tests use net.get_layers()[0] to access layer names.
• OutOfCoreNetwork requires a real file path at construction; tests create a minimal CSV via tmp_path.
• Hypothesis + capsys: Hypothesis @given tests cannot use function-scoped pytest fixtures — replaced capsys with io.StringIO + contextlib.redirect_stdout.

All tests follow existing conventions: @pytest.mark.property, @settings(deadline=None), try/except ImportError → pytest.skip guard at module level.