Skip generation when zero calls were captured; document sbt 2's cached test task

[luksow]

Closes #135.

Problem

Baklava generation runs as a post-test side effect (Tests.Cleanup). It formatted whatever serialized calls were in target/baklava/calls/ — even zero — and overwrote the previous output. Under sbt 2, bare test is an incremental task cached in a global store (~/.cache/sbt/v2) that survives clean; on a warm cache it runs 0 tests (exit 0) while the cleanup hook still fires, so a complete openapi.yml got replaced by an empty stub. Reported downstream in madrileno-dev/madrileno#113 / fixed on their side in madrileno-dev/madrileno#114 by switching to testFull, with the structural fix explicitly left to Baklava.

Changes

• Empty-capture guard (BaklavaGenerate): when zero calls were captured, skip the formatters entirely — existing output stays intact — and print a warning explaining the likely cause and pointing sbt 2 users at testFull. Serialized-call cleanup is also skipped in that case (there is nothing to clean). Covered by a new BaklavaGenerateSpec (guard skips formatters + warns; non-empty path still runs formatters).
• Docs: prominent sbt 2 warning everywhere sbt test is prescribed for generation (README, intro, installation, output-formats): use sbt testFull for complete documentation, because incremental test may run a subset or nothing and the output only covers tests that actually ran.

A partial incremental run (subset of the suite) still produces a partial spec — that can't be detected mechanically (partial output via testOnly is a feature), so it's covered by the documentation warning only.

Verification

• TDD: watched the guard test fail against the extracted always-run behavior, then implemented the guard.
• sbt +core/test green on Scala 2.13.18 (53 tests) and 3.3.7 (60 tests); core/scalafmtCheckAll clean.

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