The fetch crate is about to serve downloads whose file names, sizes and addresses come from a user-editable list,
so it checks them before touching the disk: an ordinary, unique file name (no separators, drive or stream
syntax, control characters, trailing dots, Windows device names or .part suffix), a non-zero size within the
caller's bound, a well-formed SHA-256, bounded file, mirror and attempt counts, and URLs capped at 16 KiB.
FileSpec now borrows its strings (callers with static names are unchanged) so a name read from a file needs no
leaking. download_url fetches one file from an exact address, query string included, for signed links.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
The downloader in lightcraft-segment (mirrors, resume, pinned SHA-256, pure-Rust
HTTPS) was already generic apart from four SAM 3 constants. Move it to
crates/fetch (L0, native only) so other model features can use it without
depending on the SAM 3 inference code. lightcraft-segment keeps SAM3_FILES,
DEFAULT_MIRRORS, MIRRORS_ENV and wraps mirrors(); no behaviour change. The
downloader's 13 tests moved with it.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
