chore(vue-sdk): add variation composables

[joker23]

Note

Medium Risk
New public flag-evaluation API affects UI behavior, listener lifecycle, and impression timing; risk is mitigated by broad tests but worth reviewing readiness and duplicate-eval edge cases on identify.

Overview
Adds reactive Vue composables for LaunchDarkly flag evaluation, replacing the previous scaffold TODO on the public composables barrel.

Value composables (useBoolVariation, useStringVariation, useNumberVariation, useJsonVariation) and matching detail composables share useVariationCore, which returns a readonly ref, skips client evaluation until isReady(), re-evaluates on change:<key>, context/identify, and initialization state, supports reactive flag keys (ref/getter), and tears down change listeners on unmount.

Detail variants surface LDEvaluationDetailTyped and synthesize a CLIENT_NOT_READY reason before the client is ready without calling the SDK.

Exports are wired through composables/index.ts. Tests cover provider/plugin integration, single-evaluation-on-mount (impressions), readiness/failure paths, and reactive key re-subscription; makeMockClient is adjusted for handler tracking and init-status ready behavior. plugin.test.ts adds a useBoolVariation smoke test under LDVuePlugin.

^{Reviewed by Cursor Bugbot for commit d469468. Bugbot is set up for automated code reviews on this repo. Configure here.}

[github-actions]

@launchdarkly/js-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 26365 bytes
Compressed size limit: 29000
Uncompressed size: 129044 bytes

[github-actions]

@launchdarkly/js-client-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 38739 bytes
Compressed size limit: 39000
Uncompressed size: 212244 bytes

[github-actions]

@launchdarkly/browser size report
This is the brotli compressed size of the ESM build.
Compressed size: 179579 bytes
Compressed size limit: 200000
Uncompressed size: 831422 bytes

[github-actions]

@launchdarkly/js-client-sdk size report
This is the brotli compressed size of the ESM build.
Compressed size: 31978 bytes
Compressed size limit: 34000
Uncompressed size: 114243 bytes

[joker23]

@cursor review

[cursor]

✅ Bugbot reviewed your changes and found no new issues!

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 1 additional finding.