hashsig-glue: replace Box allocs with Zig-owned placement-init pattern

[zclawz]

Addresses the review comment on #918 requesting that key-generation and signing be refactored to avoid additional Rust heap allocations, consistent with the caller-supplies-buffer pattern already used in multisig-glue.

What changed

rust/hashsig-glue/src/lib.rs

Replaced the Box::new(T) + Box::into_raw return / Box::from_raw drop pattern with a placement-init approach:

Layout queries (new):

• hashsig_sizeof_keypair / _signature / _public_key — Zig queries at runtime to know how much to allocate
• hashsig_alignof_keypair / _signature / _public_key — required alignment (C malloc always satisfies this)

Placement-init (replaces Box-returning functions):

• hashsig_keypair_generate_into(out: *mut KeyPair, ...) — writes via std::ptr::write, returns i32
• hashsig_keypair_from_ssz_into, hashsig_sign_into, hashsig_signature_from_ssz_into, hashsig_public_key_from_ssz_into — same pattern

Drop-in-place (replaces Box::from_raw frees):

• hashsig_keypair_deinit / hashsig_signature_deinit / hashsig_public_key_deinit — call std::ptr::drop_in_place, do NOT free the buffer (caller owns it)

Removed: hashsig_keypair_generate, hashsig_keypair_from_ssz, hashsig_keypair_free, hashsig_sign, hashsig_signature_free, hashsig_signature_from_ssz, hashsig_public_key_from_ssz, hashsig_public_key_free

pkgs/xmss/src/hashsig.zig

• KeyPair, Signature, PublicKey wrappers now own their storage via cAlloc/cFree (thin wrappers over std.c.malloc/std.c.free)
• C malloc always returns memory aligned to max_align_t (>=8 bytes on LP64), satisfying alignment of any Rust #[repr(C)] type in hashsig-glue
• Public API is unchanged — same generate, fromSsz, sign, verify, toBytes, deinit methods; callers in aggregation.zig unaffected
• PublicKeyCache slot semantics unchanged; deinit correctly reconstructs _buf + handle from the stored integer pointer

What this does NOT eliminate

• Inner leansig Vec<u8> data inside XmssPublicKey / XmssSignature / XmssPrivateKey — inherent to the leansig API; freed correctly when drop_in_place runs Drop
• The intermediate Vec<u8> in to_bytes() serialisation helpers — leansig API constraint; short-lived temporaries that do not escape the FFI call
• multisig-glue — already used the caller-supplies-buffer pattern for proof bytes; no changes needed

Verification

• cargo check -p hashsig-glue passes cleanly
• zig ast-check on hashsig.zig, aggregation.zig, lib.zig all pass
• No remaining callers of the removed Box-based FFI symbols in the Zig codebase

cc @GrapeBaBa

[zclawz]

Closing — superseded by #935 which targets main as requested.