* fix(uploads): reject filenames reserved for temporary staging
Guard thread and shelf upload names, validate SDK batches before copying, and preserve legacy shelf downloads. Add regression coverage and document the reserved staging namespace.
* fix(uploads): report reserved names before HTTP batch ingestion
Preflight reserved staging basenames before thread storage or sandbox setup. Return HTTP 400 with a rename hint; add HTTP and strict blocking-I/O regressions and update upload documentation.
* fix(uploads): reject Windows-style staging basenames on POSIX
Normalize backslashes before the HTTP reserved-name preflight. Add POSIX basename regressions for single and mixed batches and strict blocking-I/O coverage. Clarify the preflight scope and upload documentation.
Follow-up to the review of PR #6122; AI-assisted with OpenAI Codex.
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Co-authored-by: Willem Jiang <willem.jiang@gmail.com>