Add aggregate zero initializer

[miselin]

Summary

• add zero as a contextually typed aggregate expression
• support arrays, structs, vectors, and matrices in local, return, and global initializer contexts
• lower aggregate zero values directly to LLVM zero constants
• document the distinction between an external variable declaration and a zero-filled definition

Why

The DOOM port exposed a language-level gap after initializerless pub state declarations were corrected to remain external declarations: Haven had no concise way to request an owned, recursively zero-filled aggregate definition.

This change keeps those meanings explicit:

pub state i32[4] supplied_elsewhere;
pub state i32[4] owned_here = zero;

zero has its own CST, surface AST, and core AST form rather than overloading an empty brace initializer. It requires an expected aggregate type and rejects scalar, enum, and untyped uses.

Impact

This is additive syntax, with zero becoming a reserved Haven keyword. C imports rename conflicting identifiers through the existing keyword handling.

Validation

• dune build
• dune runtest
• parser coverage for zero
• typing coverage for valid aggregate targets and invalid scalar, enum, and untyped targets
• LLVM coverage for global arrays, structs with pointer fields, and local arrays