fix(core): harden writer and node growth paths against integer overflow

[ngoyal88]

PR Description

Summary

• Add overflow checks to growable writer resize logic in src/mpack/mpack-writer.c.
• Add overflow-safe arithmetic checks in tree parsing and node page allocation logic in src/mpack/mpack-node.c.
• Preserve existing behavior for normal inputs; fail deterministically on unrepresentable sizes.

Problem

Both writer growth and node parsing/allocation paths used unchecked size_t arithmetic in boundary-sensitive code.
With extreme input sizes, this can cause wraparound and lead to incorrect bounds decisions, allocation-size corruption, or non-terminating growth behavior.

Changes

src/mpack/mpack-writer.c

• Guard used + count before growth computations.
• Saturate doubling growth to avoid multiplication overflow.
• Return mpack_error_memory when growth target is not representable.

src/mpack/mpack-node.c

• Replace overflow-prone checks using direct addition with subtraction-based guards.
• Add checked logic for node_count + total against max_nodes.
• Add checked allocation-size calculation for child page allocation (total - 1 multiplier path).

Validation

• Control-size scenarios behave unchanged.
• Large boundary probes now fail fast and safely instead of depending on wrapped arithmetic.

Risk

Low. Changes are localized, minimal, and only affect overflow-edge arithmetic and error handling paths.