docs: add package godoc and clarify public API

Prepare the extracted library for external consumption with grouped
godoc, extension point docs, and setup requirements. Wire
ErrBadCursorString into decode paths and drop redundant Paginate*Conds
aliases.

Co-authored-by: Cursor <cursoragent@cursor.com>

README.md

@@ -138,6 +138,14 @@ edgeCursor := base.CopyWithVals(
 | `string` / UUID | `NewStringValue` |
 | `time.Time` | `NewTimestampValue` |
 
+## Extension points
+
+Custom cursor value types implement [GenericExpr] (order values) and [ExprMarshaler] (index values). See package godoc for requirements.
+
+## Errors
+
+Decode failures return `ErrBadCursorString`; use `errors.Is` to detect invalid client cursors.
+
 ## Utilities
 
 - `ExtractNodes` — nodes from a `Connection`