fix(date): harden strftime memory accounting and document security model

Move strftime memory charging into the same formatting path used for padding, enforce pre-allocation checks, and add regression tests for non-string date format PoCs. Add dedicated docs clarifying that memoryLimit is cooperative DoS mitigation and not strict heap isolation.

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

test/integration/filters/date.spec.ts

@@ -210,6 +210,18 @@ describe('filters/date', function () {
       expect(() => liquid.parseAndRenderSync('{{ d | date: f }}', { d: 'now', f: '%5000000d' }))
         .toThrow('memory alloc limit exceeded')
     })
+    it('should charge memoryLimit for array format PoC', () => {
+      const liquid = new Liquid({ memoryLimit: 50, renderLimit: 1e9 })
+      expect(() => liquid.parseAndRenderSync('{{ d | date: f }}', { d: 'now', f: ['a'.repeat(2000000)] }))
+        .toThrow('memory alloc limit exceeded')
+    })
+    it('should charge memoryLimit for object toString format PoC', () => {
+      const liquid = new Liquid({ memoryLimit: 50, renderLimit: 1e9 })
+      const huge = 'a'.repeat(2000000)
+      const f = { toString: () => huge }
+      expect(() => liquid.parseAndRenderSync('{{ d | date: f }}', { d: 'now', f }))
+        .toThrow('memory alloc limit exceeded')
+    })
     it('should clamp numeric strftime pad width', () => {
       const liquid = new Liquid({ memoryLimit: 1e7 })
       const out = liquid.parseAndRenderSync('{{ d | date: f }}', { d: 'now', f: '%50000d' })