[TS] Make empty strings falsy in symbolic conditions

[CaelmBleidd]

Fixes #423.

Stack

This PR is scoped to #423 and targets #455 / #422 at 127ecb5df35366c550a32725cf4e6af25208f673. Its head is df0b9afe4b6730b9e88a466c796fd7989b539b9a (four linear commits, no merge commit).

Change

• Apply ECMAScript string truthiness to materialized strings: an empty UTF-16 backing is false and a nonempty backing is true. Ordinary non-null objects remain truthy.
• Materialize uniquely typed string and string-literal fields with UTF-16 backing. Preserve written values and mark values beyond the configured bound explicitly unsupported.
• For a dynamic reference that could be a string but has no modeled backing, fork that possibility to an explicit unsupported path. Continue supported nonstring paths instead of treating every non-null reference as truthy.

Verification

• The direct symbolic-string regression checks empty and nonempty outcomes and replays generated witnesses in Node.js. Field regressions cover symbolic and literal string reads, writes, truthiness, equality, bound failures, and Node replay.
• TsDynamicTruthinessTest checks that unbacked strings through any are recorded as unsupported. A source-level typeof value === "object" && value !== null guard forces a nonstring reference through if (value); the test requires a TsClass witness with result 1 and replays it in Node.js. An unrelated outer false branch produces an unbacked-string witness that the test resolver cannot render; the test requires and classifies this separately, without counting it as supported. The typed-object path is checked independently.
• On JacoDB [TS PBT] Schedule assertion, coverage and hypothesis targets under one budget #399 b5e10a1a0c5d279492c373caf907f6387fff998f, the previous head 1c820b5a passed ./gradlew :usvm-ts:test :usvm-ts:detektMain :usvm-ts:detektTest -PuseLocalJacodb=/private/tmp/jacodb-ts-398-build --offline --no-daemon --console=plain --quiet: 1,110 tests, 143 skipped, 0 failures/errors, and both Detekt tasks succeeded. The final commit changes only the assertion for classified unsupported witnesses; its focused TsDynamicTruthinessTest and detektTest passed, including a forced rerun of the focused test. git diff --check passed.

These are local results. Pinned core correctness and AI code hygiene my-review passes on final head df0b9afe found no actionable defects. GitHub reports no automatic checks for this stacked PR; the final #460 stack runs manual CI separately.

Boundary

The source-level Boolean(input.value) call stops at unknown-call resolution (POINTER_TARGET_NOT_FOUND) before truthiness conversion. This PR covers directly executed condition operators.