Railbase core v0.5.15

Bug-fix release.

• Fix use-after-free on borrowed vault cursor keys — aligns railbase with vault's zero-copy cursor borrow contract (Iterate/Range hand back page-aliased bytes). Origin upsert wrote under a dangling key; cron upsert could delete the wrong row (duplicate name / lost id).
• Security & admin hardening (audit 160626) — admin invite + disable lifecycle, per-request admin-status guard, RBAC write-gates (fail-closed in prod) + transactional role swap, tenant invite for brand-new users, expanded auth/session/recordtoken test coverage.

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