Fix history indices, cursor results, lazy values, and vector lookups

[codeboost]

Summary

Fix four correctness issues in history results, cursors, nested values, and vector lookups:

• Return the zero-based committed history index so it can be passed directly to deref-at.
• Reject nested lazy sequences before vector conversion can realize them. Failed writes preserve the prior value and history.
• Return the value at a cursor's path from swap! and reset!, including scoped before/after values when history returns are enabled.
• Validate vector lookup keys and bounds before coercing indices. Invalid get keys return nil or the supplied default; nth without a default and single-argument vector invocation retain their throwing behavior. Stored nil remains distinct from a missing element.

These changes correct observable API behavior without changing the storage format.

Validation

• Regression tests reproduced all four issues before the fixes.
• Added coverage for history bookmarks across later writes, nested cursor updates, rejection without lazy-sequence evaluation, and vector lookups in read, transaction, and frozen views on memory and file databases, including reopened files.
• clojure -M:test: 254 tests, 4,266 assertions, no failures or errors.
• git diff --check passes.