Remove the memory extra: it installed neo4j with nothing behind it

[Shashankss1205]

Summary

Takes option (a) Remove it from the issue — the smaller, safer change it recommends.

• pyproject.toml: deleted the memory extra (neo4j>=5.20) and removed memory from the self-referential all aggregation. Nothing under grapharc/ imports neo4j, so the extra installed a driver and delivered no capability.
• uv.lock: regenerated — neo4j is gone from the graph and from provides-extras.
• README.md: the Memory section's closing note no longer advertises the dead extra; it now names the stores that actually exist (SQLiteMemoryStore, no extra needed; LadybugMemoryStore via the ladybug extra).
• ROADMAP.md (8.7): the "still names Neo4j with nothing importing it" note now records the removal; a future Neo4j-backed store would bring its own extra with it.

No Neo4j store is implemented here, per the issue's explicit scope.

Verification

• uv sync --all-extras --group dev resolves cleanly
• uv run pytest: 1641 passed, 12 deselected (live)
• uv run ruff check .: clean
• tests/test_packaging.py::test_the_all_extra_names_every_other_extra stays green with the extra removed from both places

Fixes #3

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