Implement native InputCapture with private EIS receiver transport

Add consent, zones and outside-edge barriers, connection-scoped leases, bounded input queues and keyboard/pointer forwarding. Restore local input on release, locking, topology or keymap changes, receiver loss and emergency escape. Preserve held-key state and distinguish finger stops from absent scroll axes.

Validation: independent review cleared; 5 native InputCapture unit tests; 32 portal tests; official D-Bus contract checks on a private bus; real installed libei receiver interoperability test. Live physical barrier and consent interaction remain to be checked in a desktop session.

portal/src/main.rs

@@ -3,7 +3,9 @@ mod bridge;
 mod capture;
 mod consent;
 mod desktop;
+mod eis;
 mod inhibit;
+mod input_capture;
 mod lockdown;
 mod parent;
 mod permissions;