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.

WorldGen.gd

@@ -22,10 +22,12 @@ const GAS_GIANT_THRESHOLD: float = 0.55
 # chance independent of biome - not a second smooth region to tune against
 # the first.
 const BLACK_HOLE_CHANCE: float = 0.05
-# #63: findable without excessive exploration - research points gate mid/
-# late skill-tree nodes and vessel research, so a player needs to run into
-# one of these reasonably often, not just as a rare curiosity.
-const RESEARCH_LAB_CHANCE: float = 0.08
+# #63/#74: findable without excessive exploration - research points gate
+# mid/late skill-tree nodes and vessel research, so a player needs to run
+# into one of these reasonably often, not just as a rare curiosity. Raised
+# from 0.08 after playtesting found that rate too scarce for how central
+# research points had become to progression.
+const RESEARCH_LAB_CHANCE: float = 0.16
 
 # A predictable early gas source for play-testing. The origin chunk is
 # deterministically normal space. Its explicit in-chunk position puts it