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/c.go

@@ -0,0 +1,50 @@
+package tests
+
+import (
+	"github.com/lukaszraczylo/llm-testbench/internal/eval"
+	"github.com/lukaszraczylo/llm-testbench/internal/testkit"
+)
+
+func registerCTests(r *testkit.Registry) {
+	r.Register(cStructSizeTest())
+}
+
+// cStructSizeWant is sizeof(struct Config) on LP64 (64-bit System V / Apple
+// ARM64 ABI: natural alignment, no packing).
+//
+// ground truth: char flag(align 1, size 1) at offset 0; double ratio
+// (align 8, size 8) needs offset divisible by 8, so it starts at offset 8
+// (7 bytes padding after flag), ending at 16; int count (align 4, size 4)
+// at offset 16 (already aligned), ending at 20; short mode (align 2, size
+// 2) at offset 20, ending at 22. The struct's own alignment is the widest
+// member alignment (8, from double), so the total size rounds up from 22
+// to the next multiple of 8: 24. c_test.go cross-checks this with cc via
+// sizeof and offsetof when the cc toolchain is available.
+const cStructSizeWant = 24
+
+func cStructSizeTest() testkit.Test {
+	prompt := `Here is a C struct:
+
+` + "```c" + `
+struct Config {
+	char flag;
+	double ratio;
+	int count;
+	short mode;
+};
+` + "```" + `
+
+Assuming the LP64 data model (64-bit, no #pragma pack, no
+non-default alignment attributes), what is sizeof(struct Config) in bytes?
+Respond with only the number, nothing else.`
+
+	return testkit.Test{
+		ID:          "c-struct-size",
+		Category:    "programming",
+		Subcategory: "c",
+		Description: "Compute sizeof() for a C struct on LP64 given its member types and default alignment.",
+		Prompt:      prompt,
+		MaxTokens:   200,
+		Eval:        eval.Numeric(eval.ExtractLastNumber[int], cStructSizeWant, 0),
+	}
+}