Solana-safe buffer abstraction to avoid large stack allocations

[xoredtwice]

This MR introduces a small buffer abstraction to avoid large fixed-size stack allocations that can cause runtime failures in Solana BPF programs due to strict stack frame limits (~4KB).

A new internal SignatureBuffer abstraction unifies two execution paths:

1. Solana builds (default feature) use heap-backed buffers to remain BPF-safe
2. Non-Solana builds use fixed-capacity buffers for maximum performance

This removes manual offset arithmetic, keeps cryptographic behavior unchanged, and makes platform constraints explicit without duplicating logic.

No WOTS+ logic or parameters were modified.

Summary of changes

• Added SignatureBuffer abstraction with a minimal, uniform API.
• Replaced offset-based writes with sequential appends
• Introduced solana feature (default) for Solana-safe behavior
• Updated and added tests to cover both feature paths

Testing

cargo test
cargo test --no-default-features
cargo test --features solana

Related issue

Closes / Related to: #11

[augmentcode]

🤖 Augment PR Summary

Summary: Introduces an internal SignatureBuffer helper to avoid manual offset arithmetic when concatenating WOTS+ public-key segments, motivated by Solana BPF stack-frame limits.
Changes:

• Adds a solana feature (default) to select the buffer backing implementation.
• Replaces segment accumulation in key generation and verification with push_slice()/as_slice().
• Updates hashing call sites to consume the unified buffer view.
• Adds tests for deterministic signatures and SignatureBuffer behavior under both feature configurations.

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[augmentcode]

Review completed. 1 suggestions posted.

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[augmentcode]

SignatureBuffer’s #[cfg(feature = "solana")] variant stores [u8; constants::SIGNATURE_SIZE] inline, which is a stack allocation and seems to contradict the PR description/goal of avoiding large Solana stack frames (and it replaces prior Vec usages with stack storage). Was the cfg intended to be inverted (heap-backed on solana, fixed-capacity on non-Solana)?

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[maekwae]

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