Research lab planet: spawn more often, block extractors, clearer feedback (#74, #75, #76)

#74: RESEARCH_LAB_CHANCE raised from 0.08 to 0.16 - research points
gate mid/late skill-tree nodes and vessel/lab research now (#63), so
0.08 was too scarce for how central they'd become to progression.

#75: an extractor placed on a research_lab planet would sit on an
empty resource id forever, since it yields nothing by design. Refused
in BuildController's placement confirm, with the same "explain the
refusal" idiom every other build gate uses. register_capture_points()
gained an optional definition_id parameter so the refusal/reward logic
can tell a research_lab planet apart without threading an extra
argument through every existing caller.

#76: conquering a research_lab awarded a research point with no
on-screen acknowledgement beyond a HUD number the player has no reason
to be watching at that exact moment. CaptureLabel's existing
"CONQUERED" flash now names the reward directly on a research_lab.

test_extractor_building.gd and test_planet_conquest_gate.gd both use a
"first planet chunk found" scan with no regard for definition_id; in
this world's fixed seed that chunk happens to be a research_lab, so
#75's new gate started refusing their otherwise-unrelated extractor
placement checks. Both now skip a research_lab chunk when picking
their target, same as test_research_lab_extractor_gate.gd does on
purpose for its own dedicated coverage of that refusal.

tests/test_planet_conquest_gate.gd

@@ -29,7 +29,12 @@ func _initialize() -> void:
 	for x in range(-15, 15):
 		for y in range(-15, 15):
 			var chunk: Vector2i = Vector2i(x, y)
-			if WorldGen.has_planet_at_chunk(chunk):
+			# #75: this test is about the conquest gate, not resource type -
+			# skip a research_lab specifically, since it yields nothing and
+			# is correctly refused regardless of conquest (see
+			# test_research_lab_extractor_gate.gd for that behaviour).
+			if WorldGen.has_planet_at_chunk(chunk) \
+					and WorldGen.planet_data_at_chunk(chunk).definition_id != "research_lab":
 				target_chunk = chunk
 				found = true
 				break