taurus v0.12.0

New API — per-node binding_wrapper (#262)

Added void* binding_wrapper to TaurusNode base struct. Language
bindings (Ruby FFI, Python, etc.) can cache their native wrapper
object on first node access, eliminating per-node FFI call overhead
on subsequent traversals.

New functions:

• taurus_node_get_binding_wrapper(node) → void*
• taurus_node_set_binding_wrapper(node, void* wrapper)

ABI change: TaurusNode grows from 12→20 bytes. Element struct
grows from 80→88 bytes. Minor version bump.

Measured impact (from #262 benchmark data):

Query	Before (Ruby)	With cache	Nokogiri
//book (100 nodes)	88 µs	~13 µs	13 µs
Union (200 nodes)	188 µs	~27 µs	27 µs

The binding eliminates 100+ FFI calls per nodeset traversal. On
first traversal, the binding wraps each node and caches the wrapper.
On subsequent traversals, the cached wrapper is found with zero FFI
calls.

The field is opaque to libtaurus — never dereferenced or freed.
Initialized to NULL on node creation.

Combined with the batch accessor (taurus_xpath_result_get_nodes,
shipped in v0.11.4), this addresses the complete #262 proposal.