test/core: cover ns form + :use; fix use-impl spec parsing

Checking test coverage for the namespace tiers surfaced a real bug nothing
exercised: use-impl checked (array? s) to find a spec's ns symbol, but a
vector spec like [src.x] is a pvec/tuple, not a Janet array — so (:use ...)
and standalone (use ...) errored. Fixed to coerce (pvec->array) then take the
head when the spec is indexed (matching require-impl).

Added coverage that was missing:
- conformance (x3 modes): the (ns …) form itself + :use refers.
- namespaces-spec: (ns …) form, :use, standalone use.

The :use/(use …) path had ZERO tests before, which is why the bug slipped
through earlier tiers.

EBNF needs no change — it's a reader/syntax grammar with no special-forms
enumeration; ns/require/protocol syntax is unchanged (the destructuring note
was already updated in jolt-f79).

Gate green: conformance 269x3, fallback-zero 38/4, self-host, sci-bootstrap,
bootstrap-fixpoint, staged-bootstrap, clojure-test-suite >=4034/67, namespace,
all unit + spec (namespaces 27/27).

test/integration/conformance-test.janet

@@ -135,6 +135,8 @@
 
    ### ---- HIGH: aliased namespace calls ----
    ["require :as alias"  "\"1,2,3\"" "(do (require (quote [clojure.string :as s])) (s/join \",\" [1 2 3]))"]
+   ["ns form + alias"    "\"HI\""  "(do (ns my.a (:require [clojure.string :as s])) (s/upper-case \"hi\"))"]
+   ["ns :use refers"     "42"      "(do (ns src.u) (def helper 42) (ns dst.u (:use [src.u])) helper)"]
 
    ### ---- MED: missing core fns ----
    ["peek vec"        "3"             "(peek [1 2 3])"]