fix: stabilize runtime follow-up regressions

site/doc/pages/to_u64.txt

@@ -1,5 +1,9 @@
 :sig
-u64 DTree::to_u64()
+u64 DTree::to_u64(u64 default_value = 0) const
+
+:params
+default_value : returned when the value is missing or cannot be parsed as a number
+return value : the value as an unsigned 64-bit integer, or `default_value`
 
 :see
 >types
@@ -7,11 +11,32 @@ u64 DTree::to_u64()
 json_decode
 int_val
 to_bool
+to_s64
 to_f64
 
 :content
-Reads a `DTree` value as an unsigned integer.
+Reads a `DTree` value as an unsigned integer. This is a read accessor: it is `const`, never creates or modifies nodes, and dereferences internal references automatically.
 
-String values are parsed numerically. Boolean values become `1` or `0`. Negative values clamp to `0`.
+String values are trimmed and parsed permissively, like `to_s64()`. Boolean values become `1` or `0`. Negative values clamp to `0`, and results above the `u64` range clamp to the maximum.
+
+`default_value` is returned when:
+
+- the node is unset or holds an empty string
+- the string does not parse as a number
+- the node is map-shaped with more than one entry, or an unresolvable reference
+
+A map-shaped node with exactly one entry unwraps to that entry's value before converting. Note that a *present* negative value clamps to `0` rather than returning the default — the default signals "missing or unparseable", not "out of range".
+
+## Example
+
+```cpp
+u64 limit = context.get["limit"].to_u64(50);
+u64 owner_id = row.get_by_path("owner/id").to_u64();
+```
 
 Use this for counts, identifiers, limits, timestamps, and other non-negative numeric values stored inside a `DTree`.
+
+## Related Concepts
+
+- PHP: `intval()` with a fallback
+- JavaScript: `parseInt(x) || fallback` (within unsigned range)