fix(arborist): keep an optional peer that shadows a version not satisfying it - #10087

Fixes #10086. Related to #9249 (same prune of extraneous optional peers, different outcome).

calcDepFlags leaves a node extraneous when it is only reachable through a peerOptional edge, so #idealTreePrune removes it after a lockfile-based install that changed the tree. That is fine when the edge just ends up missing. But when the pruned copy was nested to avoid a conflict, the edge falls through to a node further up that does not satisfy it, and the saved lockfile has an invalid edge (npm ls fails with ELSPROBLEMS, npm 10 npm ci refuses the lockfile).

Now such a node is no longer marked extraneous when the node it shadows (the same name resolved from above its parent) exists and does not satisfy the edge. An optional peer that only fills an empty slot, or shadows a version that also satisfies the edge, is still pruned as before.

Tests: a unit test in test/calc-dep-flags.js covers both cases and fails without the change. The nest when peerOptional conflicts snapshot in build-ideal-tree loses one "extraneous": true line: that test builds exactly this layout, and the nested optional peer it asserts on was flagged for removal on the next install.

End to end with the repro from the issue: after npm install --save-exact jsdom@30.0.1 the lockfile keeps the nested @noble/hashes@2.3.0 copies, npm ls reports nothing invalid, and both this branch's npm ci and npm 10 npm ci accept it. The arborist suite passes with 100% coverage, and so do the install, ci and ls command tests.