Release candidate: useid 1.0

[pyyush]

Summary

• prepares @pyyush/useid for the 1.0.0 RC train
• hardens selector grounding semantics, abstention/error shapes, docs, examples, performance budgets, release workflow, and RC handoff
• keeps package metadata at 0.2.0 until the explicit RC version-bump commit

Local verification

• npm ci
• npm run release:verify
• npm audit clean
• package dry-run

Remaining release blockers

• no 1.0.0-rc.N version-bump commit, tag, npm prerelease, or GitHub prerelease yet
• remote CI evidence for this PR is pending
• at least one external or external-like validator must run the RC
• final 1.0.0 publish remains blocked until RC and external validation gates pass