fix(server/checkin): address outstanding nondeterminisim in graph creation

[deciduously]

Publishing packages from different entry points produced different IDs for the same package (e.g., tg publish std then tg publish jq would give std different IDs).

Root causes:

1. When computing initial labels for the Weisfeiler-Leman algorithm, external pointers (references to nodes
   outside the SCC) retained ephemeral checkin graph indices instead of resolved object IDs. These indices differ based on entry point.
2. Symmetric nodes with identical content produce identical WL labels, and the sort preserves non-deterministic input order from Tarjan's SCC discovery.

Fix:

1. In checkin_graph_node_initial_label, resolve external pointers to their object/artifact IDs.
2. Use file paths as a tiebreaker when sorting nodes with identical canonical labels.

[isabella]

this falls back to the case that caused the bug where edge has the pointer to the index, we should explicitly error.