Colour code with a scanner of our own instead of the library

dev.snipme:highlights 1.1.0 found comments before it knew the language and
paired /* with */ by ordinal, so `//` in any URL commented out the rest of
its line, every Rust `#[derive(...)]` greyed out as a comment, a `#` inside
a Kotlin string swallowed the line, and `x '*/a/*'` in shell produced a span
whose end preceded its start -- the one that crashed a card holding
`-path '*/.git/*'`. None of that could be post-processed away, because
comments won over strings before the language was known.

Highlighter.kt is one left-to-right scanner: at each position it is in a
line comment, a block comment, a string, or ordinary code, and every span is
emitted by advancing an index, so spans cannot overlap, arrive out of order
or run backwards. Languages.kt is a `Rules` row per language -- comment
tokens, block comment and whether it nests, the string forms, what opens an
attribute, and the keyword set -- so a new language is a table entry. The
keyword lists came from the library's SyntaxTokens.kt (Apache-2.0, noted at
the table) so nothing that is coloured today turns plain, and RON, TOML,
fish and JSON are coloured for the first time.

HighlighterTest.kt is a new JVM unit test source set -- 24 cases, the
library's mistakes kept as regressions, plus a sweep asserting no span
escapes the code for any language on unterminated and empty input.
AGENTS.md's app line now runs :androidApp:testDebugUnitTest.

Measured on the ai-app emulator, debug build, a ~200-line Kotlin fence sent
into a sandbox session:

  before  code highlighted: 1, 101.9ms total, 101.9ms mean, 101.9ms worst
  after   code highlighted: 1,  15.0ms total,  15.0ms mean,  15.0ms worst

and a second fence in the same run took 13.9ms, so that is the steady cost
rather than class loading. stream-bench.sh after the change:

  code highlighted: 1, 12.1ms total, 12.1ms mean, 12.1ms worst
  markdown reparsed while streaming: 1329, 2130.7ms total, 1.6ms mean, 8.7ms worst
  record: one block: 131, 11.4ms total, 0.1ms mean, 0.4ms worst
  draw phase 1.21ms per frame, the transcript 0.23ms of it

transcript-bench.sh after: draw phase 1.10ms per frame, the transcript
0.49ms (place 0.48), worst place 4.3ms -- unchanged within run-to-run noise,
as expected, since the scan happens in `warm` and not while drawing.

Looked at on the emulator: a URL inside a Kotlin string, a Rust attribute
with a lifetime and a raw string, a shell line with globs and `$#`, a RON
fence and a TOML fence all colour correctly; a Bash tool card still colours
its command; a plain Python fence -- which this change had no reason to
touch -- looks as it did; an unknown language stays plain; and a fence is
plain while it streams and colours when it freezes.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

app/androidApp/build.gradle.kts

@@ -173,6 +173,15 @@ dependencies {
     implementation(libs.androidx.lifecycle.runtime.compose)
     implementation(libs.zxing.embedded)
     implementation(libs.markdown.renderer)
-    implementation(libs.highlights)
     implementation(libs.androidx.exifinterface)
+
+    // The syntax scanner (Highlighter.kt) is pure logic with no Android imports, which is what
+    // lets it be tested on the JVM: `./gradlew :androidApp:testDebugUnitTest`. The assertions are
+    // `kotlin.test`, so the tests name no framework; JUnit is what runs them.
+    testImplementation(libs.kotlin.test.junit5)
+    testImplementation(libs.junit.jupiter)
+    testRuntimeOnly(libs.junit.platform.launcher)
 }
+
+// JUnit 6 runs on the Platform, which is not Gradle's default for a Test task.
+tasks.withType<Test>().configureEach { useJUnitPlatform() }