Return outlier_fences as a Vector, not a Tuple (fixes E2E serialisability assertion) (#31)

With the malformed `@test` macro fixed in #29, the E2E suite now
actually runs its "report is JSON-serialisable" assertion — and catches
a latent bug: `descriptive_stats` returned `outlier_fences` as a
`Tuple{Float64,Float64}`, but the report contract only allows
Number/String/Bool/Nothing/**Vector**.

Emit `[lower, upper]` as a `Vector` so the value round-trips as a JSON
array. No consumer indexes it tuple-specifically (repo-wide grep). This
is the last known-red piece of the E2E suite on `main`.

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

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

Author: hyperpolymath

src/stats/descriptive.jl

@@ -101,7 +101,7 @@ function descriptive_stats(data::Vector{Float64})
         "min" => sorted[1],
         "max" => sorted[end],
         "range" => sorted[end] - sorted[1],
-        "outlier_fences" => (q1 - 1.5 * iqr_val, q3 + 1.5 * iqr_val),
+        "outlier_fences" => [q1 - 1.5 * iqr_val, q3 + 1.5 * iqr_val],
         "normality_hint" => abs(skew) < 2 && abs(kurt) < 7 ?
                             "Approximately normal" : "Possibly non-normal"
     )