- 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.
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