feat: uniform page length for Gradescope + confirm names pass through

[briacSck]

Summary

Implements Professor Chang's feedback from her first real batch.

Uniform page length (the fix)
Every student's exam PDF now comes out the same length so Gradescope's per-question template aligns without manual fixing. Three causes of variable length addressed:

• One transcript page per scan page — _render_transcript_page shrinks font to fit a single page; dense pages no longer spill to 2-3 pages and shift structure. (pdf_assembler.py)
• Batch-wide padding — every exam is padded to the longest exam in the batch with blank page pairs, so all outputs share one page count and question N lands on the same page for everyone. run_batch computes the batch max from ExamFile.page_count (no extra OCR) and threads it through. (pipeline.py, pdf_assembler.py)
• No more part-split — the 95 MB _part1/_part2 split broke uniformity; replaced with a single-file lower-DPI fallback chain (lossless → 150 → 110 → 90). Oversize files are flagged in the run report instead of split.

Names/SIDs (no change needed — verified)
The tool already keeps names/SIDs visible on the Gradescope deliverable and masks only the copy sent to Mistral. Confirmed end-to-end on a real, non-anonymized exam: the .masked.png (API-bound) had the header blacked out; the output scan page showed Name and SID clearly. The censoring seen earlier was the pre-anonymized test set, not the program.

Test Coverage

• test_transcript_forced_to_single_page — verbose transcript stays one page
• test_pads_short_exam_to_target_length — padding to a target length
• test_size_guard_stays_single_file — tiny size ceiling never splits, downsamples instead
• test_run_batch_pads_all_outputs_to_uniform_length — two exams of different physical lengths produce equal page counts (2 × max)
• Golden interleaved snapshot unchanged.

Verification

• pytest: 63 passed
• mypy: clean (19 source files)
• ruff check: clean
• Real-file mask/unmask check: passed (masked hides identity, output shows it)

Notes

• Deliverable remains interleaved scan + transcript. If a scans-only Gradescope output is preferred later, that's a small follow-up.

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