refactor(EditCompensation): consume useJobForm + useCompensationForm hooks

[serikjensen]

Migrates the Compensation flow's edit screen onto the public hook contract introduced by #1735.

Opening as a draft stacked on refactor/job-compensation-hooks-split because the migration validates the hook contract end-to-end against the existing integration tests. Will rebase onto main and unmark draft once #1735 lands.

Summary

• Replace BaseComponent + EditCompensationPresentation with a single BaseBoundaries-wrapped Root that instantiates useJobForm and useCompensationForm and threads them through composeSubmitHandler. The job-then-compensation submit chain is preserved (POST job → PUT stub comp on create, PUT job → PUT comp on update). Events (EMPLOYEE_JOB_CREATED/UPDATED, EMPLOYEE_COMPENSATION_UPDATED) and the onSaved(Compensation) contract are unchanged.
• Delete the duplicate internal compensationSchema.ts and the EditCompensationPresentation/test pair: form behavior is now covered by the hooks' unit tests (useJobForm.test.tsx, useCompensationForm.test.tsx, compose.test.tsx). The integration suite (Compensation.test.tsx) is unmodified and continues to assert screen-level behavior.
• Promote title (job) and flsaStatus/rate/paymentUnit (comp) via optionalFieldsToRequire: { update: [...] } so the screen always renders these fields as required. The hooks default these to required only on 'create'; the screen-level UX always required them.
• Render fields inline with formHookResult={...} props because the screen interleaves job and compensation fields. Keep the FLSA change warning as screen-local presentation state via useWatch on the comp form's hookFormInternals.formMethods.control. The hook still exposes data.canTriggerCarveOut for partners who want to drive their own confirmation prompt.

Hook-design fixes that landed on #1735 because of this migration

The migration revealed two contract mismatches that #1735 needed before this PR could pass the existing integration tests without modification:

1. flsaStatus validator made .optional() so the field can render its empty placeholder when nothing is provided. Schema-level requiredFieldsConfig still enforces it on submit in create mode. New fallback chain inherits from the employee's primary job's current compensation when adding a secondary job.
2. Reorder the FLSA rate validation so RATE_EXEMPT_THRESHOLD surfaces before RATE_MINIMUM for EXEMPT + low rate (matches pre-migration UX).

Test plan

• npm run test -- --run src/components/Employee/Compensation — 85/85 ✅ (integration suite untouched)
• npm run lint:check src/components/Employee/Compensation — 0 errors
• Smoke-test Employee.Compensation flow end-to-end in Storybook / sdk-app (no-jobs onboarding, single-Exempt edit, multi-job FLSA carve-out)
• Rebase onto main and unmark draft after feat!: split useCompensationForm into useJobForm + useCompensationForm hooks #1735 merges

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