feat: LLM accuracy testing framework with 17-test catalog

Framework runs a category/subcategory test catalog against multiple
LLMs through an OpenAI-compatible endpoint. Deterministic evaluators
score each response. Reports render as table, markdown, or JSON.

internal/tests/codebase.go

@@ -0,0 +1,66 @@
+package tests
+
+import (
+	"github.com/lukaszraczylo/llm-testbench/internal/eval"
+	"github.com/lukaszraczylo/llm-testbench/internal/testkit"
+)
+
+func registerCodebaseTests(r *testkit.Registry) {
+	r.Register(codeTraceGoTest())
+}
+
+// mix is the traced function for codeTraceGoTest: deterministic bit
+// manipulation plus recursion. It is called directly (not re-implemented)
+// to derive codeTraceGoWant, and codebase_test.go calls it again with the
+// same input to ground that value, per PLAN.md's instruction that this
+// test's ground truth be established by running the same function.
+func mix(n int, depth int) int {
+	if depth == 0 {
+		return n
+	}
+	if n%2 == 0 {
+		n = n >> 1
+	} else {
+		n = (n << 1) ^ 0x2A
+	}
+	return mix(n, depth-1) + depth
+}
+
+// codeTraceGoWant is mix(37, 4), computed by running mix itself.
+//
+// ground truth: mix(37,4): 37 is odd -> n=(37<<1)^0x2A=96, return
+// mix(96,3)+4. 96 is even -> n=48, return mix(48,2)+3. 48 is even -> n=24,
+// return mix(24,1)+2. 24 is even -> n=12, return mix(12,0)+1. depth=0 ->
+// return 12. Unwinding: 12+1=13, 13+2=15, 15+3=18, 18+4=22.
+var codeTraceGoWant = mix(37, 4)
+
+func codeTraceGoTest() testkit.Test {
+	prompt := `Here is a Go function:
+
+` + "```go" + `
+func mix(n int, depth int) int {
+	if depth == 0 {
+		return n
+	}
+	if n%2 == 0 {
+		n = n >> 1
+	} else {
+		n = (n << 1) ^ 0x2A
+	}
+	return mix(n, depth-1) + depth
+}
+` + "```" + `
+
+What is the exact value of mix(37, 4)? Respond with only the number,
+nothing else.`
+
+	return testkit.Test{
+		ID:          "code-trace-go",
+		Category:    "research",
+		Subcategory: "codebase",
+		Description: "Trace a recursive Go function combining bit manipulation and recursion to an exact numeric result.",
+		Prompt:      prompt,
+		MaxTokens:   200,
+		Eval:        eval.Numeric(eval.ExtractLastNumber[int], codeTraceGoWant, 0),
+	}
+}