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/spec/namespaces-spec.janet

@@ -16,6 +16,11 @@
 
 (defspec "namespaces / ns operations"
   ["in-ns switches"     "true"     "(do (in-ns 'my.ns) (symbol? 'x))"]
+  # ns is a macro over in-ns/require/use/import (Stage 2 jolt-eaa): the form sets
+  # the current ns and processes its clauses.
+  ["ns form + alias"    "\"HI\""   "(do (ns my.app (:require [clojure.string :as s])) (s/upper-case \"hi\"))"]
+  ["ns :use refers all" "9"        "(do (ns src.lib) (def helper 9) (ns dst.app (:use [src.lib])) helper)"]
+  ["standalone use"     "7"        "(do (ns src.l2) (def k 7) (in-ns 'dst.a2) (use '[src.l2]) k)"]
   ["ns-name"            "true"     "(do (require (quote [clojure.string])) (= 'clojure.string (ns-name (find-ns 'clojure.string))))"]
   ["find-ns existing"   "true"     "(some? (find-ns 'clojure.core))"]
   ["find-ns missing"    "nil"      "(find-ns 'does.not.exist)"]