Fix native protocol reorganized handler: extract header_t not height_t

[echennells]

Fixes #783

protocol_native::handle_chase's reorganized case extracted the event payload as node::height_t, but chaser_confirm::set_reorganized posts a header_link (node::header_t), as documented in chase.hpp and as the sibling organized case and do_top(node::header_t) already assume.

node::header_t (header_link::integer = uint32_t) and node::height_t (size_t) are distinct alternatives of the event_value variant, so std::getnode::height_t on a header_t-holding variant throws std::bad_variant_access. handle_chase is NOEXCEPT, so the throw becomes std::terminate: any chain reorganization while a client holds a native top subscription crashes the node.

Extract node::header_t instead, matching the emitter, the chase.hpp doc comment, the organized sibling, and do_top's signature.

Adds two regression tests to test/protocols/native/native_block.cpp:

• a non-throwing get_if guard that the reorganized payload is header_t;
• a behavioral test driving the reorganized event through the running server (mirrors native__ws_top_subscribe__progressive_notify__expected). Pre-fix the behavioral case aborts via bad_variant_access; post-fix it passes, so the fix and test land in the same commit.