Warn when sources sit under sbt/Maven layout that bleep won't pick up

[oyvindberg]

Summary

• Best-effort heuristic at bootstrap: for each project without an explicit `sources:` or `sbt-scope:`, scan for the six common sbt/Maven-style source paths (`src/{main,test}/{scala,java,kotlin}`). If any exists, contains at least one source file, and is not already in the project's resolved `sourcesDirs` → emit one `warn` per offender pointing at the file and listing three fixes (move, set `sources:`, or set `sbt-scope:`).
• Catches the exact footgun that silently neutered `KotlinIT` for months. Until Kotlin formatting via ktfmt; bump scalafmt + GJF defaults #595 lands, `KotlinIT` files at `src/main/kotlin/` are never compiled and the suite passes as a no-op. With this PR, that situation is loud at bootstrap.
• Cost: at most 6 `Files.isDirectory` probes per project, then one short-circuited `Files.walk(...).anyMatch(...)` per existing suspect dir. Zero false positives on bleep's own multi-project build.

Relationship to #595

This PR catches the layout footgun in general; #595 fixes the specific `KotlinIT` paths that exposed it. They are independent — either can land first. If #595 lands first, the suspect-paths check has nothing in this repo to warn about; if this lands first, `KotlinIT` runs will print warnings until #595 merges (tests still pass; warnings are not errors).

Test plan

• New `CheckMisplacedSourcesTest` with 8 unit cases (kotlin/java/scala warn; correct layout no-warn; empty dir no-warn; `sources:` override no-warn; `sbt-scope: main` no-warn; `src/test/` also caught). Suite passes in 628ms.
• Smoke-tested on bleep's own multi-project build — no false positives across the dozens of projects with mixed `sources:` overrides and default layouts.
• `bleep fmt --check` clean.

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