Add workflow compiling changed preprints

[tschm]

Adds .github/workflows/preprints.yml, which compiles a preprint's LaTeX only when its source folder has changed, or when it has never been compiled.

How it works

• Detect: for each preprints/*/source folder, compares its git tree hash (git rev-parse HEAD:<dir>) with the source-tree: line in the notes of the preprint-<name> release. A missing release means the preprint has never been compiled. Folders without a top-level .tex containing \documentclass are skipped (currently seven_sins).
• Compile: one matrix job per preprint. Runs make compile if there is a Makefile (with uv installed, so rice_50_digits regenerates its assets), otherwise latexmk -pdf.
• Publish: on main, replaces the preprint-<name> release with the new PDF and records the tree hash. On pull requests the PDF is only attached to the run as an artifact.
• Manual run: a force input rebuilds everything.

Because this PR changes the workflow file, it should build rice_50_digits here as a check. Merging it will publish the first release.

Testing

• actionlint passes.
• Ran the detection step locally: rice_50_digits came out as never compiled and seven_sins was skipped. An empty list gives an empty matrix and skips the compile job.
• A local make compile of rice_50_digits takes about 1m40s.

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