fix(preview): restore --output-dir for single-file preview without _quarto.yml

[cderv]

Summary

Fixes a regression introduced in v1.9.18 where quarto preview file.qmd --output-dir <dir> fails with The --output-dir flag can only be used when rendering projects. when run in a directory without _quarto.yml. This is how the Quarto VS Code extension/Positron drives single-file preview inside R packages (DESCRIPTION present, no _quarto.yml), so the bug surfaces in those workflows even though the underlying CLI behavior has nothing to do with DESCRIPTION — any quarto preview --output-dir against a single file outside a project hits it.

Fixes #14489. Reported via posit-dev/positron#13370.

Root cause

preview.ts pre-creates a singleFileProjectContext and passes it to render() as pContext. The 1.4-era synthetic-project trigger in render-shared.ts only fires when pContext is null:

if (!context && options.flags?.outputDir) {
  context = await projectContextForDirectory(path, nbContext, options);
  ...
}

Preview always supplies a context, so the trigger is skipped and the path falls through to validateDocumentRenderFlags, which throws.

quarto render foo.qmd --output-dir baz/ works because there pContext is null and the trigger fires.

Fix

Extend the trigger to also fire when pContext is a single-file context:

if (options.flags?.outputDir && (!context || context.isSingleFile)) { ... }

The discarded singleFileProjectContext cleans up through globalTempContext. The synthetic project produced by projectContextForDirectory populates files.input, so the existing isProjectInputFile arm at line 100 takes over and the path proceeds via renderProject as for any other project render.

A regression unit test exercises the exact preview pattern (tests/unit/render-shared-output-dir.test.ts).

llm-docs/synthetic-project-context.md documents the synthetic-project mechanism and the history of the two single-file context shapes (singleFileProjectContext for extensions, projectContextForDirectory for --output-dir).

Test plan

• New unit test passes (tests/unit/render-shared-output-dir.test.ts)
• Existing tests/smoke/render/render-output-dir.test.ts still passes
• tests/smoke/render/render-minimal.test.ts still passes
• Manual repro: dir without _quarto.yml + test.qmd, quarto preview test.qmd --no-browser --no-watch-inputs --output-dir <tmp> --embed-resources no longer throws and produces output

Backport

This will be backported to v1.9 separately.

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