Reuse pre-scan plan for interfile scans

[mfow-nullify]

Summary

• reuse the pre-scan SAST plan for interfile scans when the core rule set exactly matches
• thread an optional precomputed plan through CoreRunner so interfile scans can skip rebuilding the bundle/plan in memory
• add unit coverage for safe reuse, mismatch fallback, wiring from run_rules, and the no-replan CoreRunner path

Testing

• python3 -m py_compile cli/src/semgrep/core_runner.py cli/src/semgrep/run_scan.py cli/tests/default/unit/test_interfile_scan_plan.py
• PATH="/Users/mfow/.codex/worktrees/6889/opengrep/.venv311/bin:$PATH" .venv311/bin/python -m pytest cli/tests/default/unit/test_interfile_scan_plan.py -q
• PATH="/Users/mfow/.codex/worktrees/6889/opengrep/.venv311/bin:$PATH" .venv311/bin/python -m pytest cli/tests/default/unit -q

Notes

• I also ran PATH="/Users/mfow/.codex/worktrees/6889/opengrep/.venv311/bin:$PATH" .venv311/bin/python -m pytest cli/tests/default -m 'not pysemfail' -x -vv --tb=short.
• That broader default suite stops on an unrelated existing snapshot mismatch in cli/tests/default/e2e/test_autofix.py::test_autofix_json_output[imported-entity.yaml-imported-entity.py-json-dryrun].
• The diff there is an autofix snapshot expectation for $X.abstract_content and does not touch the scan/interfile code changed in this PR.