Pin the descriptor before mmap validation so guest descriptor reuse cannot
redirect the eventual mapping. Apply the canonical guest creation mask when
creating named objects, distinguishing new objects before setting permissions
without changing the host process umask. Add regressions that reproduce both
failures and document the remaining host requirements.
Constraint: Guest umask must not mutate the process-wide host mask
Confidence: high
Scope-risk: narrow
Tested: 17 targeted Linux runtime tests passed with zero skips; conventions and diff checks; Clang analyzer on three changed production translation units
Not-tested: Full project build and suite, Windows runtime, console hardware and titles
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