Validate Scala 3.9.0 LTS + Scala.js 1.22.0 on Mill 1.1.8; cut 0.3.1 (TJC-2274)

[arcaputo3]

Summary

Prerequisite for TJC-2273 (fast-mcp-scala → Scala 3.9.0 LTS). Scala 3.9.0 emits Scala.js 1.22 IR, which the linker bundled by Mill 1.1.x's ScalaJSConfigModule (IR ≤ 1.20) rejects; 0.3.0's explicit-scalaJSVersion path is the supported route, and until now the suite only proved it against Scala 3.8.2 on Mill 1.1.5.

• Scala 3.9.0 LTS in every Scala.js integration fixture, example-scalajs, examples, and the README snippets (Scala.js stays 1.22.0). No plugin code change was needed — the suite passes as-is.
• Mill 1.1.8 for the plugin build (.mill-version, launcher defaults, fixture/example headers) and, the part that actually decides which Mill the fixtures run in-process, a separate millTestkitVersion = "1.1.8". The compile target stays at millVersion = "1.1.5", so the supported floor is unchanged and the jar under test is the real consumer shape (0 "Mill version … is different than configured" warnings, previously 49).
• The plugin's own scalaVersion stays at 3.8.2 on purpose and build.mill now says why: Mill 1.1.5 and 1.1.8 both compile build files with Scala 3.8.2, and a plugin built with newer Scala emits TASTy consumers' Mill cannot read.
• Managed Bun 1.4.0 → 1.4.1 (released 2026-09-04) with checksums for all twelve release assets, verified against the release's SHASUMS256.txt; 1.4.1 writes lockfileVersion 2 so committed locks stay valid, and 1.4.0 stays in the table for explicit pins. @types/bun has not reached 1.4.1 on npm yet, so the shipped default pairs with a new DefaultBunTypesVersion = "1.4.0" instead of tracking Bun blindly (which would 404 every TypeScript frozen install on release day); modules that override bunVersion keep lockstep.
• Version sweep 0.3.0 → 0.3.1 (README, example builds) and a 0.3.1 CHANGELOG entry.

Verification (local, macOS arm64, Mill 1.1.8, managed Bun 1.4.1)

• millbun.compile, millbun.test: green
• millbun.integration (full suite, in-process Mill 1.1.8 via mill-testkit 1.1.8): green, 0 version-mismatch warnings
• example-scalajs (app.run, app.test.testForked), example-typescript (same), examples __.compile, scalafmt check: see CI
• Consumer check: fast-mcp-scala on Scala 3.9.0 + this plugin at 0.3.1 (publishLocal) compiles JVM/JS/Native and its Bun conformance suite passes 42/42 (TJC-2273 PR)

Linear: TJC-2274

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