fix: compact fragmented unsized index pages

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Fixes variable-width index corruption after delete fragmentation, restart, and further inserts. Unsized pages grow a slot directory from the front and values from the tail; after reload, physical delete fragmentation was absent from the in-memory split estimate and the regions could overlap. The fix counts slot and node-id replacement bytes, compacts fragmented pages before persistence, and covers both single-event and batch write paths. The regression reproduces 211 inserts, 37 deletes, a restart, and 20 inserts; it now also asserts persisted utility size plus tail offset never exceeds page capacity. Released beta.2 fails with InvalidSubtreePointer; this branch reopens and reads all 194 live rows. Persistence errors are terminal as of 4f1e4d2 and are returned by wait_for_ops. Verification: cargo fmt, cargo clippy --all-targets with warnings denied, targeted regression, and all 51 unsized tests. Current master CI already has tracked follow-up failures outside these paths.