fix(merge): safe cross-Qube dimension merging with translation map

[HCookie]

Summary

Fixes a critical bug where merging two Qubes built from different sources (with dimensions interned in different orders) would cross-intersect unrelated dimensions, causing panics or silent data corruption.

Problem

When two Qubes intern their dimensions in different orders, the same integer ID can refer to different dimension names. The merge logic previously compared dimensions by ID directly, meaning e.g. number coords could be intersected against time coords.

Solution

Dimension translation map

• Before merging, build a mapping from other's dimension IDs to self's by interning all of other's dimension names into self's key_store.
• All merge comparisons now go through this map.

Missing dimension handling

• When a dimension exists only in other, copy its entire subtree into self (previously silently dropped).

drop_recurse fix

• Iteratively splice out consecutive matching dimensions instead of only one level (previously could skip intermediate levels).

Supporting changes

• Add Clone for Node and Qube
• Add axes(), dimensions(), all_dim_ids(), get_or_intern_dim(), prepend() methods
• Include coordinate intersection safety fixes (return Empty instead of panicking on cross-type/empty operands)

Tests

• Test merging Qubes with different dimension interning order
• Test that shared dimensions merge correctly

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Part of the split from #102. Independently mergeable.