feat(levels): c50c trades one good idea repeated for five that alternate

c50b is sealed where it stands, as a reference sample rather than a candidate.
Its packed construction was judged good — clearing a board from the rim inward
is the experience it proved out — but at 32 of 50 levels it played as one level
fifty times: 体验太单一了，因为基本都是满盘. The note on why it is kept lives at
the top of its module, because the lesson is the useful part.

It was monotonous on a second axis nobody asked about. 环绕 and 花瓣, the two
route families the brief singles out as the good-looking ones, were absent from
it entirely — not by choice but by a crossing threshold. At 1.0 crossings per
cell, 0% of orbits boards pass and 4% of spirals; at 1.2, 84% and 94% do. The
gate meant to answer "交叉点太多" had quietly deleted the families most worth
looking at, so it moves to 1.2 — still half the 2.36 that drew the complaint.

c50c alternates five kinds of level every three to five: packed boards down to
32%, long-route departures to watch, deep locks to solve, small quick boards,
and segments given over to orbits, petals and spirals by name. All six families
appear, no family exceeds 24%, and the plan is 13 segments rather than 9.

Selection learned to ask for variety outright — by family, piece count and
longest drawn route — instead of hoping it falls out of a difficulty score.

Rule bot across all three: 50/50 cleared and zero dead taps everywhere; c50c
sits between the other two on blind rotations and on how much there is to do.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/campaign-versions.test.js

@@ -16,7 +16,7 @@ test('every selectable edition retains its own level count, order and chapters',
   // Historical editions keep their exact shape; new ones are appended, because
   // other tests address the originals by position.
   assert.deepEqual(CAMPAIGN_VERSIONS.map(c=>[c.id,c.levels.length,c.chapterSize,c.chapters.length]),[
-    ['v0.1',24,8,3],['v0.2',48,6,8],['v0.3',48,6,8],['v0.4',20,5,4],['v0.5',1000,50,20],['c50',50,10,5],['c50b',50,10,5],
+    ['v0.1',24,8,3],['v0.2',48,6,8],['v0.3',48,6,8],['v0.4',20,5,4],['v0.5',1000,50,20],['c50',50,10,5],['c50b',50,10,5],['c50c',50,10,5],
   ]);
   assert.equal(campaignVersion('v0.1').levels[0].name,'转一转，路就通了');
   assert.equal(campaignVersion('v0.1').levels.at(-1).name,'六角大满贯');