fix: align B.3 G-composition with USM path semantics

[iqdoctor]

Background

This PR addresses a residual post-#29 inconsistency, not a reopening of #29. It narrows one remaining mismatch in B.3 only.

Problem addressed

Closes #33.

B.3 still taught G_eff = SpanUnion({G_i}) as the default aggregate law, which overstates applicability for composed claims that depend on essential supports with narrower path-wise overlap.

Minimal repair

• replace the raw-union default formula for G
• state explicitly that essential dependency paths compose by intersection
• reserve SpanUnion for declared independent support lines to the same claim
• align CC-B3.2 and CC-B3.5 with the same law

Out of scope

• no wording cleanup in A.2.6
• no teaching-layer fixes in C.2 / C.3
• no unrelated edits elsewhere in B.3