Fix Add node sequential placement

[AbigailDeng]

Problem

Workflow Activity vNext called insertStepByType without an insertion predecessor. Add node therefore appended a disconnected step, left existing adjacent ordering implicit, and surfaced backend implicit_next warnings after serialization.

Solution

• Materialize only eligible adjacent implicit transitions before insertion: non-terminal steps with no explicit next and no branches.
• Insert after the selected step, or after the final valid step when nothing is selected.
• Reuse the existing insertion helper to preserve a selected step's former explicit successor.
• Keep explicit next values, branch targets, terminal behavior, server validation findings, backend semantics, graph rendering, and Team Member Workflow Studio unchanged.
• Add pure document coverage plus route-level Add node coverage through accessible UI interactions.

Impacted paths

• apps/aevatar-console-web/src/shared/studio/document.ts
• apps/aevatar-console-web/src/shared/studio/document.test.ts
• apps/aevatar-console-web/src/pages/workflow-activity-vnext/hooks/useWorkflowEditor.ts
• apps/aevatar-console-web/src/pages/workflow-activity-vnext/index.test.tsx
• Focused design and implementation documents under apps/aevatar-console-web/docs/superpowers/

Verification

Passed:

pnpm --dir apps/aevatar-console-web exec jest \
  --runInBand \
  --runTestsByPath \
  src/shared/studio/document.test.ts \
  src/pages/workflow-activity-vnext/index.test.tsx

Result: 2 suites passed, 107 tests passed, 0 failures.

Dependency-discovery preflight:

pnpm --dir apps/aevatar-console-web exec jest \
  --listTests \
  --findRelatedTests \
  src/pages/workflow-activity-vnext/hooks/useWorkflowEditor.ts \
  src/shared/studio/document.ts

Result: selected 8 test files, including task-external Team Member Workflow Studio and Studio domains. The related-test execution was intentionally rejected under docs/testing-policy.md fan-out guard; the two changed/directly-related test files above were run explicitly instead.

Passed changed-file static checks:

pnpm --dir apps/aevatar-console-web exec biome lint \
  src/pages/workflow-activity-vnext/hooks/useWorkflowEditor.ts \
  src/pages/workflow-activity-vnext/index.test.tsx \
  src/shared/studio/document.test.ts \
  src/shared/studio/document.ts

Passed repository guards:

bash tools/ci/test_stability_guards.sh
python3 apps/aevatar-console-web/docs/design-baselines/workflow-activity-vnext/verify-baseline.py
git diff --check origin/feat/2026-08-04_workflow-activity-vnext...HEAD

Full frontend suite/build: deferred to GitHub CI by personal local workflow policy.

Local full TypeScript checking was also skipped because no reliable affected-only project typecheck is available; GitHub CI owns full type verification.

Design baseline

Design baseline:
  apps/aevatar-console-web/docs/design-baselines/workflow-activity-vnext/
Primary design:
  aevatar-workflow-activity-vnext.excalidraw
Design SHA-256:
  30e74d7b410ae72c4c91432355436679033679c54c10b1702908435b001577de
Contract specification:
  apps/aevatar-console-web/docs/superpowers/specs/
  2026-08-04-workflow-activity-vnext-design.md
User paths:
  apps/aevatar-console-web/docs/superpowers/specs/
  2026-08-04-workflow-activity-vnext-user-paths.md
Authentication and localization:
  Existing Aevatar login, callback, session, returnTo, and Umi locale logic;
  presentation may change, behavior may not.
Production data source:
  Real APIs and API-acknowledged user actions only; no mock fallback.
Baseline integrity:
  python3 apps/aevatar-console-web/docs/design-baselines/
  workflow-activity-vnext/verify-baseline.py