Add dim keyword arg to enforce a fixed coordinate length per graph - #13

Summary

_BaseGraph.__init__(dim=...) declares that every embedded vertex in this graph must have a coord of exactly that length; add_vertex() checks it and raises ValueError on mismatch. Vertices added with no coord at all are unaffected regardless of dim -- there's nothing to check. Defaults to None (today's fully unconstrained behaviour).

This gives real, validated meaning to what MVTB's BundleAdjust.py has always been trying to express with pgraph.UGraph(6) -- a comment right above that call says "initialize the graph, nodes have 6D coordinates" -- but the arg positional parameter recently removed in the type-hints pass never actually did anything with that 6. It was dead code from the start; dim is the real feature that call was reaching for, now as an explicit, named, and impossible-to-confuse-with-metric keyword arg. (MVTB's call site will need a small follow-up update to UGraph(dim=6) -- tracked separately.)

Test plan

• New test_dim: valid/invalid dim values, coord-length mismatches, unconstrained (dim=None) graphs, and the coord=None passthrough case
• 45/45 tests pass
• Sphinx docs build cleanly; runblock demonstrates the happy path only, matching this file's existing convention of not featuring deliberate failures in executed examples (the ValueError case is documented via :raises: and prose instead)
• mypy: no new errors (baseline unchanged at 6, all in the pre-existing plot()/highlight_vertex cluster)