[cmj0121]

Closes #358.

What

x in xs over a list now asks each element the == it has. A declared type goes through its Eq, and a
container, a tuple or an array compares by its parts — the comparison #295 built and in did not call.
E9061 retires.

print [3] in [[1, 2], [3]]      # true
print (2, "y") in [(1, "x")]    # false
print P(2, "b") in ps           # true, through P's Eq

Decisions made here

• The value looked for is on the left of each comparison — x in xs asks x.eq(e), and stops at the
  first match. It only shows with a hand-written eq; the corpus case prints the order.
• A map or a set with an aggregate key answers E9052, not a new code. They hash rather than compare, and
  an aggregate has no Hash here. That half of E9061 was never an equality gap, and E9052 is the number a
  map built with such a key already had.
• An unnamed element with a part of no equality is refused for that part, under the code == gives it
  (E4020, E4038, E4034), in the words of in.

Verified

• Local gates, one at a time, each read: build fixpoint lint fmt-self error-codes-check seed-gaps refuse reject
  mem-check oracle corpus sanitize-corpus mem-peak examples docs-mirror docs-links docs-zerg docs-refused
  docs-repeat marker-codes chapter-codes deviation-check refusal-cites method-gaps gates treesitter dead-code
  stdlib-test compiler-test check-equal reject-fuzz fmt-corpus behaviour conformance productions suites desugar.
  refuse, reject and mem-check also under /bin/bash.
• The new mem-check case was red with the value's give-back removed (5850 per round), green after.
• The abort path was red by hand: with the two holds removed, leaks --atExit on the corpus case reports one
  leak; none with them.

Not verified

• make test as one board was not run.
• make lsp was not run: it fails on this machine for the reason in zrt_listdir sorts by insertion: a large directory costs its square, and make lsp fails under a crowded $TMPDIR #361, which this change does not touch.

🤖 Generated with Claude Code

https://claude.ai/code/session_01N8jd1oK2orM6diHxhvWhC7