observability: export by_node and tokens_by_model from grapharc.observe

[Shashankss1205]

Summary

grapharc.observe re-exports most of its public surface from grapharc/observe/__init__.py, but two functions that grapharc/observe/cost.py defines and documents are missing from it: by_node and tokens_by_model. Callers have to reach past the package and import from the submodule directly.

Why this matters

Reading a run afterwards is half of what makes graph engineering auditable rather than hopeful — if a function that answers "which node spent this?" is only reachable by knowing the submodule path, people stop asking the question. attribute and attribute_thread are already exported; these two belong beside them.

Where in the code

• grapharc/observe/__init__.py — the from grapharc.observe.cost import (...) block and __all__
• grapharc/observe/cost.py — where by_node and tokens_by_model are defined

Confirm the gap:

grep -n "by_node\|tokens_by_model" grapharc/observe/__init__.py   # currently no hits
grep -n "^def by_node\|^def tokens_by_model" grapharc/observe/cost.py

What to change

1. Add by_node and tokens_by_model to the import block in grapharc/observe/__init__.py.
2. Add both names to __all__, keeping the list's existing alphabetical grouping.
3. Add a short test asserting they are importable from the package root, so the gap cannot reopen.

How to verify

./.venv/bin/python -c "from grapharc.observe import by_node, tokens_by_model; print('ok')"
uv run pytest tests/test_replay.py -q
uv run ruff check .

Acceptance criteria

• from grapharc.observe import by_node, tokens_by_model works
• Both names appear in __all__
• A test covers the import, and fails if either export is removed again
• uv run pytest stays green and uv run ruff check . is clean

Skill level — good first issue

A genuinely small, self-contained change: two lines of imports, two __all__ entries, and one test. It is a good way to see how this repo expects every change to arrive with a test that would fail without it. Ask here if anything is unclear and someone will help you through it.