[TS] Concatenate symbolic strings

[CaelmBleidd]

Closes #424.

Symbolic string concatenation copies UTF-16 code units between the same private backing arrays used by string inputs, equality, typed fields and witness extraction. It preserves operand evaluation order and supported concrete primitive conversions; results above maxArraySize produce explicit unsupported paths.

String-only unions inferred for expressions such as (value && "a") + "!" are accepted. The empty-input branch returns "!", and the nonempty branch returns "a!". A regression covers this union in either operand of +, checks both input branches and explicit unsupported outcomes, and replays generated witnesses in Node.js.

Based on main at a8c423a75025d8fea0dfba4144d3af9626df2581, after #456 merged. Current head: 9493596b599fbe4ce22cf2f3f4edd22f851b824a. The PR contains the concatenation commit and its string-union fix, with three changed files.

Local validation on this head with the published pinned JacoDB dependency, without a local substitution: all 5 concatenation tests passed with zero skips/failures/errors, including Node.js replay. The new source-level regression failed on the previous head ae79f17adb7407aeee5a48b052eaddd24a5b613a because the empty-input property was missing, then passed with the fix. Detekt main/test passed with zero code smells; git diff --check passed. The broader 32-test string validation belongs to the previous rebased head ae79f17ad. Local Java 23 and Node.js 26 were used; fresh GitHub CI on the current head is a separate gate.

Symbolic Number-to-string conversion, object ToPrimitive, and genuinely unbacked string references remain unsupported. No merge performed.