[TS] Make empty strings falsy in symbolic conditions

[CaelmBleidd]

Fixes #423.

Empty strings now evaluate to false in conditions, !, &&, and ||; nonempty strings and ordinary objects remain truthy. Uniquely typed string fields receive bounded UTF-16 contents, literal field types preserve exact code units, and reads preserve prior writes. Dynamic string references without modeled contents produce an explicit unsupported outcome while supported nonstring paths continue.

This version contains only #456's changes on top of merged #455 at 2c4f17c4c9fc0d70057e374ec462600a0d9c4bbf. Both new backing readers use the private mkStringBackingLValue. Regression tests follow the current discoverProperties structure and replay generated inputs in Node.js. The guarded-any test now requires the replayable strings provided by #455 instead of expecting a missing-backing extraction failure.

Validation on head f2a22c368c515e3591c8f5f0fd02a1be19eedf23, using published pinned JacoDB without substitution:

• Full :usvm-ts:test: 988 passed, 142 skipped, 0 failures/errors.
• :usvm-ts-calls:test: 13 passed, 0 skipped/failures/errors.
• :usvm-ts:detektMain, :usvm-ts:detektTest, and git diff --check: passed.
• Source regressions cover empty/nonempty inputs, string fields, literal fields, unconditional/conditional writes, and explicit unsupported outcomes; replay checks run in Node.js.

The model remains bounded by TsOptions.maxArraySize. Values outside that contract are not evidence of unbounded string support. This review covers directly executed condition operators; source-level Boolean(...) call resolution was not validated.

These are local results. CI for the published replacement head must be checked after publication.