[FEAT] Capabilities, protocol handshake server half (pairs with plugin protocolVersion 1) :

- src/capabilities.js: PLUGIN_FEATURES gate keyed on the plugin's monotonic protocolVersion + isUnknownRouteResult() covering every era of unknown-route signature (legacy 404, old-plugin enveloped error, failed ticket)
- Discovery captures protocolVersion/pluginVersion from ping across all three sources (registry validation now reuses the ping body, port scan, default port); unity_list_instances surfaces pluginVersion
- unity_component_batch_wire degrades gracefully on plugins without the route: executes entries as single component/set-reference calls with a degraded marker instead of failing the batch
- Idea credit: community PR #32 (D3vCrow), re-implemented on current main with reactive detection

[FIX] Schemas : referenceInstanceId in batch-wire entries was still type number - the one site the 2.28.3 64-bit string sweep missed; now a decimal string

[FEAT] Tests : capabilities unit suite (gate + signature matrix), batch-wire degrade protocol test (2 entries -> 2 single calls, order preserved), pluginVersion surfacing test - 39/39 green