[WIP takeover] Checkpoint local Cosmos runtime patches - #163

WIP takeover checkpoint — do not merge together with takeover/cosmos-runtime-patches-20260721. Both branches have the same final tree; this branch preserves the dirty main workspace as one snapshot commit.

What this checkpoints

• explicit tokenizer cache-directory handling
• optional cached text embeddings in the local video dataset
• inclusive selection of the last valid video window
• mixed-temporal-length cropping in both visualization callbacks

Commit 5fd267a725a004d47eb58a4a25d38ed39852dabc matches all four dirty working-tree blobs exactly. The original local main pointer remains at upstream a2c298b0a3df3778b973fe65e9e58877b292d8a7 and its four modifications remain untouched.

Validation

• PASS: AST parsing of all four changed Python files
• PASS: Ruff check and format-check on all four files
• PASS: gitleaks scan of the four-file diff
• PASS: largest changed file is 18,052 bytes; no cache or ordinary-Git oversize file is included
• BLOCKED: no focused unit tests exist for these four paths
• BLOCKED: the local pytest harness rejects configured option --suppress-no-test-exit-code, indicating the matching pytest plugin/environment is absent

Handoff

Prefer the three-commit alternative branch for code review because it preserves intent per change. Keep this PR only as the exact dirty-workspace checkpoint unless staff explicitly chooses the squashed history.