feat: harden scope writes, iteration, and readSize (#898)

Block writes to dangerous keys in assign/capture/increment/decrement, use own-property Symbol.iterator for plain objects when ownPropertyOnly is true, fix inherited size reads, and sanitize filter iteration scopes.

Co-authored-by: Cursor <[email protected]>

docs/source/tutorials/options.md

@@ -138,9 +138,7 @@ It defaults to `false`. For example, when set to `true`, a blank string would ev
 
 **lenientIf** modifies the behavior of `strictVariables` to allow handling optional variables. If set to `true`, an undefined variable will *not* cause an exception in the following two situations: a) it is the condition to an `if`, `elsif`, or `unless` tag; b) it occurs right before a `default` filter. Irrelevant if `strictVariables` is not set. Defaults to `false`.
 
-**ownPropertyOnly** hides scope variables from prototypes, useful when you're passing a not sanitized object into LiquidJS or need to hide prototypes from templates. Defaults to `true`.
-
-Built-in DoS limits and host isolation guidance are documented in [Security Model](./security-model.html).
+**ownPropertyOnly** limits template property reads on plain scope objects to own properties (no inherited prototype keys). Defaults to `true`. Blocked keys (`__proto__`, `constructor`, `prototype`) always apply. [`Drop`][drop] values, iteration, `.size`/`.first`/`.last`, filters, and custom tags follow separate rules—see [Security Model](./security-model.html).
 
 {% note info Nonexistent Tags %}
 Nonexistent tags always throw errors during parsing and this behavior cannot be customized.
@@ -163,3 +161,4 @@ Parameter orders are ignored by default, for example `{% for i in (1..8) reverse
 [jekyllInclude]: /api/interfaces/LiquidOptions.html#jekyllInclude
 [raw]: ../filters/raw.html
 [outputEscape]: /api/interfaces/LiquidOptions.html#outputEscape
+[drop]: /api/classes/Drop.html