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.

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)"])