Documented initializer torture test needs

[Copilot]

The issue targets improving pcc’s handling of large, mixed designated/positional initializers used in real-world code. This PR description reflects the intended changes but no code updates were applied in this session.

• Initializer torture test (planned)

  • Add a C99 initializer torture case covering mixed designated/positional fields, nested anonymous structs, and function pointers in cc/tests/c99/initializers.rs.

• Parser/IR alignment (planned)

  • Validate designated initializer ordering and offset handling in cc/ir/linearize.rs.

Example initializer structure intended for the test:

struct Node { int a; struct { int x; } anon; int (*fn)(int); };
struct Node t = { .anon = { .x = 1 }, .a = 2, .fn = foo };

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.