Add submission validation script

[mvanhorn]

Adds validate_submission.py - a standalone pre-submission checker that catches the most common issues from #677 before participants open a PR.

Usage:

python3 validate_submission.py records/track_10min_16mb/2026-03-24_MySubmission/

What it checks:

• Folder is under records/track_10min_16mb/ or records/track_non_record_16mb/
• Required files present: README.md, submission.json, train_gpt*.py, and at least one train log
• submission.json has required fields (val_bpb/mean_val_bpb, name/run_name, date) with correct types
• Training script has valid Python syntax
• Total folder size reported (warns near 16 MB cap)

Example output (current SOTA):

Validating: records/track_10min_16mb/2026-03-23_LeakyReLU_LegalTTT_ParallelMuon

  [PASS] Folder location: Track: track_10min_16mb
  [PASS] Required files: README.md, submission.json, train_gpt.py
  [PASS] Train log: Found: train_seed1337.log, train_seed2025.log, train_seed42.log
  [PASS] submission.json: OK
  [PASS] Training script syntax: OK (train_gpt.py)
  [PASS] Size check: Total size 0.14 MB

Result: PASS (6 passed)

Tested against all 25 existing records in the repo. 21 pass clean, 4 flag known issues (genuinely missing files or fields in those specific submissions).

Uses only Python stdlib - no extra dependencies. 263 lines.

This contribution was developed with AI assistance (Claude Code).