* build: standardize the Debug build dir on build/debug
`docs/BUILD.md` and `AGENTS.md` document `./setup.sh Debug` producing
`build/debug`, but `setup.sh`/`setup.ps1` created `builddir` and `.clangd`
pointed there too, so the documented layout and the actual tree disagreed.
Align the Debug mapping with the documented layout:
- setup.sh / setup.ps1: Debug -> `build/debug`
- CMakeLists.txt: Debug `_meson_build_dir` -> `build/debug`
- .clangd: CompilationDatabase -> `build/debug`
- plugin test fallbacks, S3 smoke default, quality-gate candidate list, and
doc examples -> `build/debug`
Legacy `builddir` trees stay readable: the quality gate and local-ci scripts
still probe `builddir` as a fallback, and `plugin_trust` already treats
`/build/` paths as build artifacts.
* build: migrate remaining Debug consumers off builddir
Follow-up to the Debug build-dir standardization: tracked consumers still
hardcoded `builddir` and would break or silently skip against a fresh
`build/debug` tree. Migrate them, keeping tolerant fallbacks where a legacy
tree may still exist.
- CI: `.github/workflows/tests.yml` runs `./setup.sh Debug` and then compiled
`-C builddir`; switch BUILD_DIR/COMPILE_DB_DIR/compile paths to `build/debug`.
Same for windows-test-diagnose.yml.
- IDE: `.vscode/tasks.json` build/test/bench tasks target `build/debug`.
- Runtime: `object_storage_plugin_loader` also probes `build/debug/plugins`
(and the flat `build/debug`), retaining the `builddir` fallbacks.
- Tests/docs: ONNX tuning integration plugin dir, benchmarks, and plugin
READMEs/examples use `build/debug`.
`./build/*` is already excluded from the quality-gate find and `.dockerignore`,
so no exclusion lists change.
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