feat(ui): ImplantPending — diegetic Layer-1 "generating" indicator (#960)

A reusable implant-layer busy indicator (Araminta's spec): an animated
bracket sweep `[ >>>>···· ]` with a caption, themed entirely via ImplantTheme.
The Atlas shows it centered over the canvas while the Layer-1 proxy is Pending
and hides it on Ready — so cache hits never flash it and serverless mode never
shows it.

Layer separation (per direction): ImplantPending belongs to the diegetic
implant UI only. The global, non-diegetic UI layer must use its own busy
indicator with its own visuals — sharing the sweep logic is fine, resting on
this theme/these glyphs is not. Documented in the component header.

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

client/tests/test_atlas_overlays.gd

@@ -23,3 +23,11 @@ func test_generation_data_round_trips() -> void:
 	}
 	v.set_generation_layer1(mock)
 	assert_that(v.get_generation_layer1()).is_equal(mock)
+
+
+func test_implant_pending_start_stop() -> void:
+	var p: ImplantPending = auto_free(ImplantPending.new())
+	p.start("GENERATING LAYER 1")
+	assert_bool(p.visible).is_true()
+	p.stop()
+	assert_bool(p.visible).is_false()