feat(client/sdk-utils): additive support for @percy/playwright-dropin baseline seed + sync verdict

[Sriram567]

Summary

Two additive, backward-compatible changes in support of the new @percy/playwright-dropin drop-in SDK (which reroutes Playwright toHaveScreenshot() through Percy). No behavior change for existing SDKs/flows.

• @percy/client createBuild gains optional parallelNonce / parallelTotal / source params. These let the drop-in's first-build baseline-seed path create a separate parallel build with its own deterministic nonce — engaging percy-api's named-lock dedup independently of the head build. When unset, they fall back to the env-derived parallel identity and source exactly as before. The only two existing callers (core/src/snapshot.js, cli-build/src/finalize.js) pass none of them → unchanged.
• @percy/sdk-utils postComparison now surfaces response.body.data (the sync-cli per-comparison verdict / {error}) when the global .percy.yml snapshot.sync is on, so the drop-in's sync classifier can read the verdict. Non-sync responses omit data → fire-and-forget unchanged.

Why

The drop-in (separate package) needs (a) a deterministically-nonced baseline build to seed first-build diffs across CI shards, and (b) the per-comparison verdict for opt-in sync-assertion mode. Both are implemented as minimal, additive hooks rather than forking the head/snapshot flow.

Testing

• @percy/client: new createBuild param specs (override-set + env-fallback paths); 24 existing createBuild specs unchanged/green; @percy/cli-build 84/84 green (createBuild signature unaffected).
• @percy/sdk-utils: postComparison change covered; 168 tests, 100% NYC coverage maintained.
• ESLint clean on both changed files.

Post-Deploy Monitoring & Validation

• What to monitor/search
  • Logs: existing @percy/client build-create / postComparison debug logs — confirm no new error rate.
  • Metrics: build-create success rate unchanged for non-drop-in SDKs.
• Validation checks
  • Run any existing SDK (e.g. @percy/cypress) build to confirm createBuild/postComparison behave identically (no params passed).
• Expected healthy behavior: existing SDK builds create + upload exactly as before; only the drop-in passes the new params.
• Failure signal / rollback trigger: any change in build-create behavior for existing SDKs → revert this PR (additive-only, safe to revert).
• Validation window & owner: one release cycle; Percy SDK owner.

Note

This is the CLI half of a 3-repo change (percy-api + the @percy/playwright-dropin package). Per KD10, these packages must publish before the drop-in works against published @percy/*.

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