PC version override: decouple PC artifact version from build target

[rochala]

What

A launch-time knob to load the presentation compiler from a different scala3-presentation-compiler_3 version than the build target's own Scala version — and the classloader/dependency work needed to actually make such a PC load. Useful for testing a locally-built dotty PC against a real project.

The override (commit 1)

• PcVersionOverride — resolved once at startup from system properties (env fallback):
  • -Dsls.pc.version=<v> / SLS_PC_VERSION — global default for every module.
  • -Dsls.pc.versions=<module>=<v>,… / SLS_PC_VERSIONS — per-module map, keyed by build-target display name, taking precedence over the global.
  • Resolution per module: per-module entry → global default → the target's own Scala version (unchanged behaviour when nothing is set).
• PresentationCompilerProvider.get resolves the effective version per module and logs (slf4j) once per PC creation when an override is active.
• Coursier's default ivy2Local repo means a publishLocal-ed snapshot resolves directly.

Making it actually load (commit 2)

The override alone wasn't enough — a PC built against a different Scala version than sls's bundled compiler failed to load. Three coupled fixes:

• PcParentClassLoader — the PC's URLClassLoader used the host app loader as parent, so dotty.tools.* (e.g. InteractiveDriver) leaked from sls's bundled scala3-compiler (via scala3-tasty-inspector) while dotty.tools.pc.* loaded from the override jars → NoSuchMethodError. The new parent has the bootstrap loader as parent and shares only the host↔PC bridge packages (scala.meta.pc, org.eclipse.lsp4j, com.google.gson), forcing dotty.tools.* and the stdlib to load from the PC's own jars.
• build.mill — bump mtags-interfaces 1.6.3 → 1.6.7 so the shared scala.meta.pc interface has the methods newer PCs call (e.g. sourcePathMode).
• ServerImpl — 1.6.7 pulls a newer lsp4j where Diagnostic.getMessage is Either[String, MarkupContent]; a chimney transformer unwraps it to the String that lsp.Diagnostic.message expects.

Note for review

Commit 2 globally bumps the host's mtags-interfaces (and transitively lsp4j) to make a newer PC line work. That's an intended dependency move, but worth a sanity check that 1.6.7 is the right floor.

Why module-keyed

A project has multiple modules; pinning the PC per module (with a global fallback) lets you isolate debugging to one target in a cross-built project. The key is the build-target display name, logged on PC creation so it's discoverable.

Future work

The override is currently static (read at startup). The design supports making it runtime-updatable via an LSP endpoint (workspace/didChangeConfiguration or a custom command) with no restart — hold it in a Ref, update on the endpoint, call invalidateCompilers(). Not in this PR.

Test

PcVersionOverrideSpec covers map parsing (incl. malformed entries) and the precedence chain. ./mill sls.test is fully green.

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