BRIDGE_REGISTER_MODEL/BRIDGE_REGISTER_ACTION built their generated static
registrar names by token-pasting the model/action type directly onto a
fixed prefix (bridge_model_reg_##M, bridge_action_reg_##M##_##A). That only
produces a valid identifier when both arguments are bare identifiers -- a
namespace-qualified type pastes ':' characters into the token and fails to
compile, with the error pointing at the macro expansion rather than the
call site.

Key the generated names on __LINE__ instead (via the standard two-level
CAT indirection so it expands before pasting). There is exactly one
registration per macro invocation, so __LINE__ is equally unique, and it
no longer depends on how the type is written. Unqualified registrations
keep compiling unchanged.

Fixes #21

Signed-off-by: Yaraslau Tamashevich <yaraslau.tamashevich@gmail.com>

BRIDGE_REGISTER_MODEL/BRIDGE_REGISTER_ACTION keyed their generated
anonymous-namespace registrar variables on __LINE__ to avoid pasting a
namespace-qualified or template type into the identifier (issue #21).
__LINE__ is only unique within a single physical file, though: two
different headers that each invoke one of these macros on the same line
number produce the same generated identifier once both are transitively
#include'd into one translation unit, which is a hard redefinition error
since C++ unnamed namespaces are per-TU, not per-file. This is exactly
what broke the WASM demo build in CI (multiple bank model headers, each
registering on the same line number, pulled into one autogen TU).

Switch the key from __LINE__ to __COUNTER__, which increments
monotonically across the whole translation unit regardless of which file
expands it, so it cannot collide this way.

Add tests/test_registration_same_line.cpp (with companion headers
issue21_same_line_{a,b}.hpp, whose BRIDGE_REGISTER_MODEL invocations are
deliberately pinned to the same physical line number) as a regression
test; confirmed it fails with the redefinition error under __LINE__ and
passes under __COUNTER__. Updated docs/spec/core/registry.md with the
naming-scheme rationale.

Signed-off-by: Yaraslau Tamashevich <yaraslau.tamashevich@gmail.com>