test: move pure core unit tests from react-doctor/tests to core/tests

Per AGENTS.md: 'packages/core/tests/ — service tests + run-inspect
orchestration tests; packages/react-doctor/tests/ — CLI +
end-to-end fixture tests'.

Seven test files were testing pure core helpers from the wrong
package (long-standing architectural drift; tests imported from
@react-doctor/core to exercise core internals):

- dedupe-diagnostics.test.ts
- evaluate-suppression.test.ts
- errors.test.ts
- filter-for-surface.test.ts
- find-jsx-opener-span.test.ts
- format-error-chain.test.ts
- parse-tailwind-major-minor.test.ts

These have no CLI dependency, no fixture-directory dependency, and no
end-to-end concerns. Move them into packages/core/tests/. Imports
keep using @react-doctor/core (core supports self-imports — see the
existing tests/run-inspect.test.ts).

74 tests migrated; total test count unchanged (185 + 1478 = 1663
matches the pre-move 111 + 1552 = 1663).

Larger fixture-dependent tests (discover-project.test.ts,
find-monorepo-root.test.ts, etc.) stay in place for now — moving them
requires either moving the fixtures directory or threading parent-
relative paths through the tests. Documented as a follow-up.

Co-authored-by: Aiden Bai <aidenybai@users.noreply.github.com>

Author: cursoragent