CDX bottom navigation

DbGoBottom() / AdsGotoBottom() on a CDX order now reaches the rightmost B-tree leaf directly instead of walking every leaf from the left. The index header is refreshed first so a changed peer root is not ignored. Empty leaves at the right edge are skipped toward the last live key.

The change retains the existing key/record-number ordering, duplicate-key behavior and index cursor interface. An empty or fully erased tree still has no positioned key. A malformed branch that exceeds the page or reaches the 64-level traversal limit returns an error rather than walking indefinitely.

This is the CDX navigation work developed from Pritpal Bedi’s B_BIG trace and carried in the bedipritpal/OpenADS mtfix41 release. It does not change record locks, physical record-count policy, the Harbour RDD core, the wire protocol, network security settings or build/release workflows.

Regression coverage includes a multilevel tree with 20,000 duplicate keys, a peer append after a reader has opened the index, backward movement from the last key, erased rightmost leaves, an empty index and a fully erased tree. No quantified latency or page-read improvement is claimed by these tests.