llm: LlmCallConfig + callConfigEquals + deepFreeze — the stateless request vocabulary

The call configuration (model + sampling scalars) becomes named
vocabulary: per-conversation state that the session log records as part
of the request header (the reconstructability RFC on this branch), with
callConfigEquals as the real-change detector behind logged header deltas
and deepFreeze as the ownership helper the loop applies to every built
request. dsh-llm stays stateless — request in, chunks out; no
conversation object lives here.

packages/llm/llm/tests/call-config.spec.ts

@@ -0,0 +1,44 @@
+/**
+ * call-config unit tests: field-wise LlmCallConfig equality (the real-change
+ * detector behind logged header deltas) and the deepFreeze ownership helper
+ * the loop applies to every built request.
+ */
+
+import { describe, expect, it } from 'vitest'
+import { callConfigEquals, deepFreeze } from '../src/call-config.ts'
+
+describe('callConfigEquals', () => {
+  it('compares every field, including the stop list element-wise', () => {
+    expect(callConfigEquals({ model: 'm' }, { model: 'm' })).toBe(true)
+    expect(callConfigEquals({ model: 'm' }, { model: 'x' })).toBe(false)
+    expect(callConfigEquals({ model: 'm', temperature: 0.5 }, { model: 'm' })).toBe(false)
+    expect(callConfigEquals({ model: 'm', maxTokens: 1 }, { model: 'm', maxTokens: 2 })).toBe(false)
+    expect(callConfigEquals({ model: 'm', stop: ['a'] }, { model: 'm' })).toBe(false)
+    expect(callConfigEquals({ model: 'm', stop: ['a'] }, { model: 'm', stop: ['a', 'b'] })).toBe(false)
+    expect(callConfigEquals({ model: 'm', stop: ['a'] }, { model: 'm', stop: ['b'] })).toBe(false)
+    expect(callConfigEquals({ model: 'm', stop: ['a', 'b'] }, { model: 'm', stop: ['a', 'b'] })).toBe(true)
+  })
+})
+
+describe('deepFreeze', () => {
+  it('freezes nested structure in place and returns the same reference', () => {
+    const value = { a: { b: [1, { c: 'x' }] } }
+    const frozen = deepFreeze(value)
+    expect(frozen).toBe(value)
+    expect(Object.isFrozen(value)).toBe(true)
+    expect(Object.isFrozen(value.a)).toBe(true)
+    expect(Object.isFrozen(value.a.b)).toBe(true)
+    expect(Object.isFrozen(value.a.b[1])).toBe(true)
+    // ESM runs in strict mode: mutation throws rather than silently failing.
+    expect(() => { (value.a.b[1] as { c: string }).c = 'y' }).toThrow(TypeError)
+  })
+
+  it('passes primitives through and terminates on cycles', () => {
+    expect(deepFreeze(42)).toBe(42)
+    expect(deepFreeze(null)).toBeNull()
+    const cyclic = { self: undefined as unknown }
+    cyclic.self = cyclic
+    deepFreeze(cyclic)
+    expect(Object.isFrozen(cyclic)).toBe(true)
+  })
+})