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/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;