docs: reorganize into en/ru and add English translation

docs/en/modules/03-reactive.md

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+# Reactivity Module: `src/interpreter/reactive.rs`
+
+Dependency tracking system between variables and VDOM elements.
+Allows recalculation of only changed elements (Phase 3).
+
+---
+
+## `ElementId(u32)`
+
+Unique element identifier in the VDOM tree. Assigned by `ReactiveTracker::alloc_id()`
+during template loading.
+
+```rust
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub struct ElementId(pub u32);
+```
+
+**Used as:**
+- Key in `dirty_set: HashSet<ElementId>`
+- Key in `dependencies: HashMap<ElementId, HashSet<String>>`
+- Field in `Element::element_id` (VDOM → tracker link)
+- Source for `iced::widget::Id` (Phase 9.2: `format!("ti:{}", id.0)`)
+- Source for scrollable_id (`el.element_id.0 as u64`)
+
+---
+
+## `ReactiveTracker`
+
+Dependency graph: variable → list of elements that reference it.
+
+```rust
+pub struct ReactiveTracker {
+    subscribers: HashMap<String, HashSet<ElementId>>,
+    dependencies: HashMap<ElementId, HashSet<String>>,
+    dirty_set: HashSet<ElementId>,
+    next_id: u32,
+}
+```
+
+**Fields:**
+- `subscribers` — for each variable: which ElementIds depend on it
+- `dependencies` — for each element: which variables it depends on (reverse mapping)
+- `dirty_set` — elements to recalculate in the next frame
+- `next_id` — counter for `alloc_id()`
+
+### Methods
+
+| Method | Description |
+|--------|-------------|
+| `new()` | Empty tracker |
+| `alloc_id() -> ElementId` | Allocate a new ID, increment counter |
+| `add_dependency(element, var_name)` | Register a dependency |
+| `scan_value(element, value)` | Scan a string for `$var` and add dependencies |
+| `on_variable_changed(name)` | Mark all dependent elements as dirty |
+| `take_dirty_set() -> HashSet<ElementId>` | Take dirty_set and clear it |
+| `is_dirty(id) -> bool` | Check if an element is marked dirty |
+| `reset()` | Clear all data |
+
+### scan_value()
+
+Parses a string for `$var_name` patterns:
+
+```rust
+"Hello $name, you are $age years old"
+// → add_dependency(element, "name")
+// → add_dependency(element, "age")
+```
+
+Used during template loading for each properties string containing `$`.
+As a result, each element knows which variables it depends on.
+
+### Update cycle
+
+```
+Event → update() → on_variable_changed("var")
+  → dirty_set = {element_A, element_B, ...}
+  → take_dirty_set() → evaluate_vdom_incr(roots, &dirty_set)
+    → for dirty_set elements: recalculate
+    → for others: return as-is
+```
+
+---
+
+## Tests
+
+| Test | What it checks |
+|------|---------------|
+| `test_basic_dependency_tracking` | Two elements, two variables, correct dirty_set |
+| `test_scan_value` | Parse `$var` from a string |
+| `test_scan_no_vars` | String without `$` creates no dependencies |
+| `test_take_dirty_set` | `take_dirty_set()` clears the internal set |
+| `test_reset` | `reset()` clears everything |