Fix keep-md cleanup deleting the markdown output when output-file has an .html extension

[cscheid]

Closes #14669.

Problem

Pairing an html format with a markdown format while output-file has an .html extension (e.g. output-file: index.html with format: html + commonmark, as nbdev sets up for llms.txt workflows) names the markdown output index.html.md — exactly the path the keep-md intermediate convention (<input-stem>.<base-format>.md, executionEngineKeepMd) assigns to the html format. The html render's cleanup removed what it took for a stale intermediate, deleting the freshly written markdown output:

• website projects: the render then failed at the output-move step (NotFound … rename '<proj>/index.html.md' -> '<proj>/_site/index.html.md'), leaving index.html and site_libs/ stranded in the project root
• default-type projects: exit 0, markdown output silently missing
• keep-md: true: no crash, but the html render's intermediate silently fought with the markdown output for the same file

Diagnosis was confirmed by instrumenting renderCleanup and watching the filesystem at 2ms resolution: the commonmark output is written and deleted 29ms later by the html format's cleanup.

Fix

Principle: keep-md intermediate handling must never write to or delete a path that a declared format owns.

Commit 1 (core fix): compute the projected output paths of the formats being rendered (projectedOutputFile, next to formatOutputFile in core/render.ts), attach them to each RenderContext, and

• skip the keep-md deletion in renderCleanup when the conventional keep-md path is another format's output
• skip the keep-md intermediate write on the same collision, with a warning (the real output wins; warn rather than error for now, may revisit)

Commit 2 (partial-render protection, split out for review): a partial render (quarto render --to html) must not delete the markdown twin a previous full render produced (outputs live in place in default-type projects). Since renderContexts already resolves every declared format (inactive ones are marked active: false), the implementation is just dropping the active-only filter from commit 1's computation — one line plus a test.

Tests

New smoke tests in tests/smoke/render/render-output-file-collision.test.ts (fixtures under tests/docs/output-file-collision/):

1. website project renders both outputs into _site, no root debris
2. default-type project keeps the markdown output
3. --to html partial render preserves an existing twin (commit 2)
4. keep-md: true warns, and the twin holds the commonmark output rather than the intermediate

Each commit's tests verified to fail without that commit's change. Genuine stale-intermediate cleanup still works (keep-md: true render followed by keep-md: false removes the leftover). Together with #14670, the full nbdev/llms.txt scenario from #14667 now works end-to-end (verified on a local merge of both branches: render succeeds and the served page's index.html.md link resolves with HTTP 200).

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[cscheid]

(Claude says:)

Note on the 8 failing windows checks (analysis after re-running them — they fail identically across 3 attempts):

• The failing tests are unrelated to this PR: babel/TinyTeX package-auto-install docs (13633.qmd, latex-hyphen-lang-{zh,es}.qmd), caption-footnotes, the feature-format matrix, and one search-index test. The new render-output-file-collision tests pass on both platforms, and every ubuntu job is green.
• Root symptom: TeX Live's fmtutil.pl/tlmgr aborts with kpsewhich -var-value=TEXMFROOT failed / 'kpsewhich' is not recognized. The PATH it reports has its first two entries joined by : (the unix separator) instead of ;, merging both TinyTeX bin dirs into one invalid entry — this happens inside TeX Live's perl, far below anything this PR touches (keep-md path bookkeeping).
• Why this PR and not Fix index.html clean-URL rewrite mangling alternate-format links on index pages #14670 (same runner image windows-2025-vs2026 20260628.158.1, same fresh TinyTeX v2026.07 download, green 36 minutes earlier): this PR adds a smoke test file, which reshuffles the deterministic test-bucket sharding; Fix index.html clean-URL rewrite mangling alternate-format links on index pages #14670 added only playwright files, which don't. A latent test-ordering-dependent TinyTeX corruption (e.g. bucket 10 runs render-pdf-svg-conversion / render-verapdf before the babel doc) would fire deterministically on exactly this PR's bucket layout — matching the 3-for-3 rerun failures while main and Fix index.html clean-URL rewrite mangling alternate-format links on index pages #14670 stay green.

I couldn't bisect the specific polluting test without a windows box; happy to file the CI investigation as a separate issue if this diagnosis looks right to maintainers.