Backed CP HiCache tail splits were floored during match, but write preparation and radix insertion could still observe the raw sub-page prefix. That could skip one token during pre-forward reservation or call CpHiCacheNodeMetadata.split() with a non-page boundary. The shared helper now floors backed partial-tail prefixes before probe, match, and insert.
Constraint: CP HiCache uses page-granular physical ownership while radix keys may keep valid-tail lengths.
Rejected: Let probe and insert keep token-precise tail prefixes | it reintroduces half-page ownership through a different entry point.
Confidence: high
Scope-risk: moderate
Directive: Any CP radix path that can split or reserve against backed HiCache metadata must use the same page-floor policy.
Tested: Remote red tests first for probe and insert failures in g0034 container.
Tested: Remote py_compile for hiradix_cache.py and test_cp_hicache_metadata.py.
Tested: Remote targeted C7 tests: 4 passed, 3 warnings.
Tested: Remote pytest test_cp_hicache_metadata.py test_cp_hicache_load_back_owner_lanes.py: 94 passed, 5 warnings.
Not-tested: Live ETE traffic with concurrent divergent tail prompts.
Co-authored-by: OmX <omx@oh-my-codex.dev>
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