feat(Combinatorics): designs exist - #521
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`Design` takes `blocks : Set (Set X)`, so designs here are simple. When
`r = q` a block of size `q` containing an `r`-set `s` must equal `s`, so
`{b ∈ blocks | s ⊆ b}.encard ≤ 1` and no design with `lam ≥ 2` exists,
while `DivisibilityCondition n q q lam` holds vacuously because every
factor is `(q - i).choose (q - i) = 1`. That makes `keevash` and
`keevash_resolvable` false at `q = r`, e.g. `q = r = 1`, `lam = 2`.
Both sources state the theorem for `q ≥ r ≥ 1` but also define a design
as a set of blocks, so they have the same corner case; it is read out
informally. Use the strict inequality, matching `keevash_sah_sawhney`
below. `keevash_large_set` is unaffected: at `q = r` its extra hypothesis
reads `lam ∣ (n - r).choose 0 = 1`.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011hxUxCg6joVz4hMa3m5Zh5
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Main now requires `group`, `status`, `visible`, `statement_revision` and `tags` in every problem manifest (EvalTools/Markers.lean, added in 07848f4). Without them `validate-manifest` fails with "missing required key: group". Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_011hxUxCg6joVz4hMa3m5Zh5
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