docs: control-logic array+scalar locals (Value model, statevars, array nodes)

docs/TECHNICAL_SPEC.md

@@ -376,9 +376,9 @@ now produces **index-aligned** CSV: each configured array becomes a column (cust
 labels supported), rows aligned by element index. Array statevars and all array nodes
 round-trip through the panel XML (`<statevar type="array" elem=… sizing=… capacity=…>`).
 
-> **Note:** This is Phase 1 (panel logic, client-side TypeScript). The equivalent array
-> support in the server-side control-logic engine (`internal/controllogic`) is a separate
-> Phase 2 effort and is not yet implemented.
+> **Note:** The above is Phase 1 (panel logic, client-side TypeScript). The equivalent array
+> support in the server-side control-logic engine (`internal/controllogic`) is Phase 2 and has
+> now landed — see the **Array-valued local variables** subsection of §3.9 below.
 
 ### 3.9 Control Logic Engine
 
@@ -389,6 +389,26 @@ and writes results back to signals. Graphs are persisted by a store and managed
 `/api/v1/controllogic`; each mutation calls `Engine.Reload()` to apply changes live. Graphs
 can be individually enabled/disabled.
 
+**Array-valued local variables (Phase 2).** Engine locals now carry a `Value` — the tagged
+union `Value = float64 | []Value` (`internal/controllogic/value.go`), the Go port of the
+panel-logic `ArrVal` model, with booleans represented as `1`/`0` at the leaves. A graph may
+declare `statevars`: each has a `Name`, a `Type` (`number` | `bool` | `array`), an `Initial`
+expression, and — for arrays — a `Sizing` policy and `Capacity`. The three sizing policies
+mirror the frontend: **dynamic** (unbounded up to `ARRAY_MAX = 1_000_000`, oldest dropped
+FIFO past the cap), **capped** (length kept ≤ `Capacity`, oldest dropped FIFO), and **fixed**
+(exactly `Capacity` elements — truncated or zero-padded). Sizing is enforced on write. State
+vars are persisted in the graph's store JSON and seeded into locals at compile time.
+
+The expression evaluator (`internal/controllogic/expr.go`) is value-polymorphic: it supports
+array literals `[a, b, c]`, indexing `arr[i]` (negative indices wrap from the end), and a
+table of array functions: `len`, `sum`, `mean`, `slice`, `concat`, `reverse`, `sort`, `scale`,
+`add`, `sub`, `push`, `set`, `insert`, `remove`, `pop`, `shift`, `indexOf`, `contains`, `fill`
+(plus `min`/`max`, which accept either scalars or an array). These are pure — they produce new
+values and never mutate a stored local. Five action nodes write back to a declared array
+statevar: `action.array.push`, `…set`, `…remove`, `…pop`, and `…clear`. The embedded Lua block
+remains **scalar-only**: reading an array local from Lua yields `NaN`. CSV export
+(`action.export`) is panel-logic-only and is **not** available in control logic.
+
 The engine also holds a `*confmgr.Store` (injected via `NewEngine`) for five config action
 nodes: `action.config.apply` resolves an instance + its set and runs `confmgr.Apply` with a
 broker-backed, audited write closure; `action.config.read` resolves a single parameter value