chore: Migrate the main build to sbt 2.x

[xerial]

Why

The main build ran on sbt 1.x. The blockers are now gone:

• sbt-scalajs-crossproject + sbt-scala-native-crossproject → sbt-uni-crossproject (feature: Add minimal sbt 2.x crossproject plugin (sbt-uni-crossproject) #623)
• third-party scala-js-env-playwright → uni-jsenv-playwright (feature: Add uni-owned Playwright Scala.js JSEnv + sbt 2.x plugin #613)
• sbt-revolver (unused in the build, no sbt 2.x release) → sbt-uni's uniRestart/uniStop/uniStatus

What

• project/build.properties → sbt 2.0.1; metabuild plugins swapped (see table in the ADR). Dropped unused sbt-buildinfo and the now-built-in addDependencyTreePlugin; bumped sbt-pack 0.23 → 1.0.0.
• Because sbt-uni-crossproject reproduces the crossProject / CrossType.Pure API, the project definitions are unchanged. Only sbt-2.x source adjustments:
  • root → uni-root — sbt 2.x derives output dirs from the project name, which collided with the uni library project ("Overlapping output directories").
  • %%% → %% everywhere; the platform-less scalajs-test-interface_2.13 named directly with a single % (in a JS project, any cross-version injects the unwanted _sjs1).
  • Test / jsEnv wrapped in Def.uncached(...) (JSEnv isn't serializable; sbt 2.x caches settings).
  • import scala.language.implicitConversions for core.jvm used as ProjectReference; xs: _* → xs*; infix classifier → .classifier(...).
• Docs: CLAUDE.md + the book's cross-build install snippet updated to the sbt 2.x / sbt-uni-crossproject approach. New ADR adr/2026-06-30-sbt2-main-build-migration.md.
• CI needs no change — the repo's ./sbt runner reads build.properties (→ 2.0.1) and code_format uses the kept sbt-scalafmt.

Validation (all on sbt 2.0.1, run locally)

Platform	Result
JVM (projectJVM/test)	1655 passed, 4 pending · +39 netty/book
JS / NodeJSEnv	uniJS ~1420 · testJS 71 passed
JS / Playwright Chromium (domTest)	374 passed
Native (projectNative/test)	uniNative 1417 (+9 pending) · testNative 71 passed

scalafmtCheckAll and pnpm docs:build pass. (uni-core reports 0 tests on every platform — it has no src/test, not a regression.)

🤖 Generated with Claude Code

[gemini-code-assist]

Code Review

This pull request migrates the main build from sbt 1.x to sbt 2.0.1. It replaces unported third-party plugins with custom, uni-owned sbt 2.x plugins (such as sbt-uni-crossproject, uni-jsenv-playwright, and sbt-uni), updates dependency syntax (switching from %%% to %% and wrapping non-serializable jsEnv instances in Def.uncached), and renames the root project to uni-root to prevent output directory collisions. Relevant documentation and an ADR have also been added or updated. I have no feedback to provide as there are no review comments.

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