Phase 1 sub-commit 5: Qt transport types

MercureClient is a single-topic SSE subscriber: opens a long-lived GET
on the hub URL with the topic query and Accept: text/event-stream,
parses the line protocol into update(data, id) signals, and reconnects
with 1s→2s→…→30s exponential backoff on drop. Tracks lastEventId across
reconnects and sends it as Last-Event-ID so the hub can replay missed
messages — backing the "Sleep / wake" path in PLAN.md §3 *Edge cases*.
One client per topic by design; multi-topic aggregation is Phase 2.

RestClient.qml is a Promise-style XMLHttpRequest wrapper. Auto-attaches
an RFC4122-v4 Idempotency-Key to every non-GET request (PLAN.md §4 and
§7) so retries are safe by default. Maps application/problem+json error
bodies into structured rejections for downstream UI.

Standalone CMake build remains green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

framework/qml/CMakeLists.txt

@@ -29,6 +29,10 @@ qt_add_qml_module(php_qml_bridge
         src/BackendConnection.cpp
         src/SingleInstance.h
         src/SingleInstance.cpp
+        src/MercureClient.h
+        src/MercureClient.cpp
+    QML_FILES
+        qml/RestClient.qml
 )
 
 target_include_directories(php_qml_bridge PUBLIC src/)