feat(strictness): subs validates bounds; assoc! bounds-checks vector index

- subs requires a string and validates 0<=start<=end<=count (no Janet
  from-end/clamping); negative/out-of-range/nil indices throw
- assoc! on a transient vector bounds-checks the index (0..count)

subs 11-fail -> 24/5 (5 remaining are byte-vs-codepoint Unicode, platform);
assoc_bang 32/6 -> 35/3. clojure-test-suite pass 3889->3898.
spec: string/subs-strictness (7), transient/assoc!-bounds (4). jpm test green.

src/jolt/core.janet

@@ -1614,10 +1614,20 @@
 
 (def core-subs
   (fn [& args]
-    (case (length args)
-      2 (string/slice (args 0) (args 1))
-      3 (string/slice (args 0) (args 1) (args 2))
-      (error "Wrong number of args passed to: subs"))))
+    (when (not (or (= 2 (length args)) (= 3 (length args))))
+      (error "Wrong number of args passed to: subs"))
+    (let [s (args 0)
+          start (get args 1)]
+      (when (not (string? s)) (error (string "subs requires a string, got " (type s))))
+      (let [len (length s)
+            end (if (= 3 (length args)) (args 2) len)]
+        # Clojure validates bounds (no negative/from-end/clamping like Janet):
+        # 0 <= start <= end <= (count s).
+        (when (not (and (number? start) (number? end)
+                        (= start (math/floor start)) (= end (math/floor end))
+                        (>= start 0) (<= start end) (<= end len)))
+          (error "String index out of range"))
+        (string/slice s start end)))))
 
 # ============================================================
 # I/O — minimal wrappers
@@ -3346,7 +3356,10 @@
 (defn- tr-assoc! [t k v]
   (tr-check-active! t)
   (case (t :kind)
-    :vector (let [a (t :arr)] (if (= k (length a)) (array/push a v) (put a k v)))
+    :vector (let [a (t :arr)]
+              (when (not (and (number? k) (= k (math/floor k)) (>= k 0) (<= k (length a))))
+                (error (string "Index " k " out of bounds for assoc! on a transient vector of length " (length a))))
+              (if (= k (length a)) (array/push a v) (put a k v)))
     :map    (put (t :tbl) (canon-key k) @[k v])
     (error "assoc! expects a transient vector or map"))
   t)

test/integration/clojure-test-suite-test.janet

@@ -18,7 +18,7 @@
 
 # Baseline: assertions Jolt currently passes across the suite. Raise as Jolt
 # improves so a regression (previously-passing assertion breaking) is caught.
-(def baseline-pass 3885)
+(def baseline-pass 3895)
 # A file is "clean" when it ran with zero failures AND zero errors.
 (def baseline-clean-files 45)
 # Per-file wall-clock budget (seconds). Normal files finish in well under 1s;

test/spec/strings-spec.janet

@@ -33,3 +33,13 @@
   ["replace"            "\"hexxo\""  "(do (require (quote [clojure.string :as s])) (s/replace \"hello\" \"l\" \"x\"))"]
   ["reverse"            "\"cba\""    "(do (require (quote [clojure.string :as s])) (s/reverse \"abc\"))"]
   ["index-of"           "2"          "(do (require (quote [clojure.string :as s])) (s/index-of \"hello\" \"l\"))"])
+
+# subs validates bounds like Clojure (no Janet from-end/clamping).
+(defspec "string / subs strictness"
+  ["subs basic"         "\"bcd\""  "(subs \"abcde\" 1 4)"]
+  ["subs to end"        "\"cde\""  "(subs \"abcde\" 2)"]
+  ["subs start>end"     :throws    "(subs \"abcde\" 2 1)"]
+  ["subs negative"      :throws    "(subs \"abcde\" -1)"]
+  ["subs end past len"  :throws    "(subs \"abcde\" 1 6)"]
+  ["subs nil start"     :throws    "(subs \"abcde\" nil 2)"]
+  ["subs on nil"        :throws    "(subs nil 1 2)"])

test/spec/transients-spec.janet

@@ -90,3 +90,9 @@
   ["conj! no args"         "[]"    "(persistent! (conj!))"]
   ["conj! identity"        "[1 2]" "(conj! [1 2])"]
   ["conj! map merges map"  "{:a 1, :b 2}" "(persistent! (conj! (transient {:a 1}) {:b 2}))"])
+
+(defspec "transient / assoc! bounds"
+  ["assoc! existing idx"  "[1 9 3]" "(persistent! (assoc! (transient [1 2 3]) 1 9))"]
+  ["assoc! at count grows" "[1 2 3]" "(persistent! (assoc! (transient [1 2]) 2 3))"]
+  ["assoc! out of bounds" :throws   "(assoc! (transient [0 1 2]) 4 4)"]
+  ["assoc! negative"      :throws   "(assoc! (transient []) -1 0)"])