Fix invalid @test message call in e2e_test.jl (#29)

`test/e2e_test.jl:112` used `@test (cond) "message"`, but Julia's
`@test` macro does not accept a fail-message argument — this raised
*"invalid test macro call"* at macro-expansion time, aborting the
**entire** E2E suite before any assertion executed. Pre-existing on
`main`; surfaced while finishing the rename.

Fix: compute the serialisability predicate, `@warn` with the offending
`key`/type on failure (preserving the original diagnostic), then `@test`
the predicate.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

test/e2e_test.jl

@@ -109,8 +109,10 @@ using Statistikles
         # Every value must be a JSON-serialisable primitive (Number, String, Bool, Nothing)
         for (key, val) in report
             @test key isa String
-            @test (val isa Number || val isa String || val isa Bool || val === nothing ||
-                   val isa Vector) "Key $key has non-serialisable type $(typeof(val))"
+            serialisable = val isa Number || val isa String || val isa Bool ||
+                           val === nothing || val isa Vector
+            serialisable || @warn "Non-serialisable report value" key type=typeof(val)
+            @test serialisable
         end
 
         # Key statistical assertions on the well-known [1..10] dataset