Model string and text value constructs used by Bastion #57

Problem

Reference workload: /Users/teakowa/Repos/wrightkit/.task-scratch/bastion/bastion.ow with scenario .../bastion/scen.json (run: see .task-scratch/bastion-loop-state.md). After athena#54 (PR #55) Bastion stops with not_modeled: "value ifThenElse" at tick 0. Constructs in this issue come from counting what Bastion uses; the PR body must list which ones it modeled and which it left NotModeled.

Scope: strings and text

String Split, Char In String, Icon String, Ability Icon String, Input Binding String, Hero/Hero Of constants, Color constants, Map constants, Button constants not yet handled, Custom String forms with all placeholder slots and localization-free rendering. Display-only strings may render a documented placeholder text only if workshop-rs/findings define it; otherwise they stay NotModeled.

Non-goals

Vectors, positions, distances: world geometry is not modeled (docs/architecture.md). A separate needs-decision issue covers them when the Bastion run reaches them.

Acceptance criteria

• A test per construct family with a boundary case (empty string, out-of-range index, missing placeholder).
• Standard gates: fmt, clippy -D warnings, tests, wasm32 build; update docs/simulator-interface.md in the same PR. A construct without a passing capture fixture is recorded in docs/findings.md as modeled-not-measured (AGENTS.md). The Bastion run must move past every construct listed here that is pure value logic; record the next stop in the PR body. Budget: split the PR if it cannot finish in 30 minutes: the first PR may cover the first half of the list.