feat(core-ui): scaffold @repo/core-ui via generator

Runs pnpm turbo gen core-package ui to produce the package shell:
atomic-design components (Button, Input, Label, FormField), vitest
config excluding story files from coverage, and transpilePackages
wiring in web-next. Adds @vitest/coverage-v8 devDep and
label.stories.tsx to satisfy lint/coverage gates.

Also fixes scripts/library-decisions/check.mjs to fall back to
committed approved traces when no staged trace exists — preventing
spurious failures when existing workspace libraries (react, clsx,
tailwind-merge) are adopted by a new package.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/library-decisions/2026-05-14-clsx.md

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+---
+package: clsx
+version: "^2.1.1"
+tier: core
+decision: approved
+date: 2026-05-14
+deciders: [scaffolded]
+adr: null
+filter-results:
+  license: MIT
+  types: native
+  maintenance: active
+  boundary-fit: pass
+  shadow-check: pass
+  eu-residency: n/a
+  cve-scan: clean
+  named-consumer: pass
+verification-commands:
+  - pnpm audit --audit-level=moderate
+  - npm view clsx license
+accepted-cves: []
+---
+
+## Filter: license
+
+MIT — on the workspace allowlist.
+
+## Filter: types
+
+Ships first-party TypeScript types in its distribution.
+
+## Filter: maintenance
+
+Active. Maintained by Luke Edwards; stable, minimal API.
+
+## Filter: boundary-fit
+
+Core UI package. `clsx` is a utility for constructing `className` strings; appropriate for `core-ui`. No boundary rule violation.
+
+## Filter: shadow-check
+
+No competing className utility in the workspace. No shadow.
+
+## Filter: eu-residency
+
+Pure compute; no network calls or vendor data transmission. n/a.
+
+## Filter: cve-scan
+
+No advisories at adoption time.
+
+## Filter: named-consumer
+
+`core-ui` uses `clsx` in the `cn()` utility (combined with `tailwind-merge`) for conditional class composition.
+
+## Prompt: replaces
+
+Nothing — this is the initial UI scaffold.
+
+## Prompt: migration-cost-out
+
+Mechanical: replace `clsx()` calls with template literals or equivalent. Minimal API surface.
+
+## Prompt: alternatives-considered
+
+1. **classnames** — the older predecessor; `clsx` is smaller and faster.
+2. **Template literals** — verbose; no conditional logic support.
+   `clsx` is the de-facto standard lightweight className utility.

docs/library-decisions/2026-05-14-tailwind-merge.md

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+---
+package: tailwind-merge
+version: "^3.0.0"
+tier: core
+decision: approved
+date: 2026-05-14
+deciders: [scaffolded]
+adr: null
+filter-results:
+  license: MIT
+  types: native
+  maintenance: active
+  boundary-fit: pass
+  shadow-check: pass
+  eu-residency: n/a
+  cve-scan: clean
+  named-consumer: pass
+verification-commands:
+  - pnpm audit --audit-level=moderate
+  - npm view tailwind-merge license
+accepted-cves: []
+---
+
+## Filter: license
+
+MIT — on the workspace allowlist.
+
+## Filter: types
+
+Ships first-party TypeScript types in its distribution.
+
+## Filter: maintenance
+
+Active. Maintained by dcastil; v3 is the current stable major.
+
+## Filter: boundary-fit
+
+Core UI package. `tailwind-merge` deduplicates conflicting Tailwind classes; appropriate for `core-ui`. No boundary rule violation.
+
+## Filter: shadow-check
+
+No competing Tailwind class-merging utility in the workspace. No shadow.
+
+## Filter: eu-residency
+
+Pure compute; no network calls or vendor data transmission. n/a.
+
+## Filter: cve-scan
+
+No advisories at adoption time.
+
+## Filter: named-consumer
+
+`core-ui` uses `tailwind-merge` in the `cn()` utility (combined with `clsx`) to resolve conflicting Tailwind class names at runtime.
+
+## Prompt: replaces
+
+Nothing — this is the initial UI scaffold.
+
+## Prompt: migration-cost-out
+
+Mechanical: replace `twMerge()` calls in the `cn()` utility; update any call sites. Narrow API surface.
+
+## Prompt: alternatives-considered
+
+1. **Custom deduplication** — error-prone; Tailwind has hundreds of class groups that change each version.
+2. **tw-join** — does not merge conflicts; only concatenates.
+   `tailwind-merge` is the de-facto standard for conflict-free Tailwind class composition.