client-core: port the ANSI parser, syntax highlighter and markdown scanner

Ports app/.../Ansi.kt, Highlighter.kt, Languages.kt and MarkdownSyntax.kt
to client-core, module for module, with every HighlighterTest and
AnsiTest case ported alongside (49 tests total). ansi.rs replaces
Compose's AnnotatedString/SpanStyle with a plain StyledText/Style pair
so the crate stays free of any UI framework, per RUST.md.

cargo test (49 passed), clippy --all-targets and fmt clean.

Co-Authored-By: Claude Sonnet <noreply@anthropic.com>

client-core/Cargo.toml

@@ -0,0 +1,38 @@
+[package]
+name = "client-core"
+version = "0.1.0"
+edition = "2024"
+
+# The app's pure logic, held once instead of twice: the event model (shared
+# with `server/` via `event-model`), the REST + SSE clients for its HTTP
+# surface (see `server/src/routes.rs`'s module doc for the table), the
+# transcript fold and cache, the markdown block model, the syntax
+# highlighter and the ANSI parser. See CLIENT_CORE.md at the repo root for
+# what this holds today, what it does not yet, and how it corresponds to
+# the Kotlin it replaces.
+#
+# No UI framework dependency of any kind -- this crate is meant to outlive
+# whichever one the app ends up drawing with (see RUST.md).
+
+[dependencies]
+event-model = { path = "../event-model" }
+serde = { version = "1", features = ["derive"] }
+serde_json = { version = "1", features = ["float_roundtrip"] }
+# The blocking HTTP client for the REST calls and the long-lived SSE GETs.
+# `server/` already depends on ureq for its own outbound HTTPS (the usage
+# poll in usage.rs) and it is rustls-backed like the rest of this project's
+# TLS, so this reuses that choice rather than pulling in reqwest's async
+# stack -- a client that runs one blocking request at a time, the way
+# Api.kt's `HttpURLConnection` calls and Sse.kt's blocking read loop do, has
+# no need of an async runtime, and RUST.md's brief for this port is
+# "lightweight" throughout.
+ureq = { version = "3", features = ["json"] }
+# Verifying the server's pinned self-signed leaf against the CA `wg-app-link`
+# mints, the same way `ServerConfig.kt`'s `applyPinnedTls` does. rustls
+# rather than native-tls because ureq is already rustls-backed here and the
+# rest of this project's TLS goes through rustls too.
+rustls = { version = "0.23", default-features = false, features = ["ring", "std", "tls12"] }
+rustls-pemfile = "2"
+
+[dev-dependencies]
+tempfile = "3"