fix: corrects peer dependency flag propagation

[owlstronaut]

Summary

Fixes peer dependency flag propagation in npm's dependency resolution system by correcting how "peer": true flags are calculated and applied.

Problem

Peer dependency flags were inconsistently and incorrectly calculated, leading to incorrect or missing "peer": true flags in the ideal tree, which could cause dependency resolution issues.

#8431 revealed a number of bugs, the worst of which appears to be that many packages in an ideal tree were marked peer when they shouldn't have been. If they were also optional, they were being removed by this pruning. This is my attempt to make a forward-fix instead of revert the aforementioned correct but also (through no fault of its own) disruptive PR #8431 .

This doesn't solve the problem of legitimate peerOptionals being uninstallable even with npm i <peer-optional-package>. It both makes sense for that to be pruned, but also for people that do it to have it either install or warn them. Right now it silently moves along. We could allow it to not be pruned that 1 time by using explicitRequests, but would subsequently be pruned on further installs.

Related:
#8464
#8431
#8489

[owlstronaut]

@G-Rath @jenseng Would love your take on this if you find time. I'm attempting to fix-foreward rather than revert, but this stuff is pretty meaty.

[jenseng]

@owlstronaut I've confirmed that this fixes my simple repro here ... npm install succeeds, and the lockfile no longer marks those deps as "peer": true 🥳

[jenseng]

The fix also works in our internal repos whose installs failed under npm@11.5.x 🚀

[wraithgar]

Thank you so much for reviewing this @jenseng it really helps when folks with domain knowledge do this.

[wraithgar]

Just wanted to double check some of these and the very first one is good. This is being correctly flagged as a peer, from npm explain @babel/core@7.28.0:

peer @babel/core@"^7.0.0" from @babel/helper-module-transforms@7.27.3