[iStorry]

Finishes download control after #17. There are three new optional settings on download, one commit each.

What's new

• connections: int (244a3fa) — the first connection asks for Range: bytes=0-. When the server answers 206 with the file's size, the file is split into parts of at least 64 KiB, with at most 16 connections. Each part is fetched by its own connection into its own place in the .part file, and each connection opens the file separately. Each outlet has room for its part's end. Pausing records how far each part got, and resuming opens a connection for each unfinished part. A 200 answer, or one with no size, keeps one connection.
• name: string (9fb959d) — this goes through the same sanitising as a name from the address (now name::safe_file_name), then Partial::create, so it can't leave the folder and never overwrites a file. "", the default, names the file after the address.
• headers: [string] and retries: int (993b2d8) — each header is written Name: value and sent on every request through http::get. A malformed header, or Range, fails the download before it asks the server anything. Retries only cover transient errors: a broken body read, a connect timeout, or a 5xx. The wait starts at 1 s and doubles up to 30 s, using recv_timeout on the event receiver so pause and cancel still work during it. While every unfinished part is waiting, status shows Retrying….

Every surface

• Checker table rows, HIR, interpreter, and compiled ABI: DownloadDesc gains name, connections, headers and retries, and ABI_VERSION goes from 4 to 5 (bumped once for all three).
• In codegen, download now holds each borrowed setting across all later expressions, not just the next one (new hold_across).
• Docs: functions.md, cheat sheet, syntax decisions 25–28 in language.md, and the roadmap (download control moves to Supported).
• The VS Code grammar test covers the new labels. The downloader.nf fixture gains a Connections slider, retries: 3, and Retrying… in is_running.

Tests

All tests run against the local test server and are deterministic, with no sleeps. The test server now supports closed ranges, bytes=a-b.

• Interpreter:
  • splitting into parts, checking the range each part asked for
  • falling back to one connection when the server can't send parts
  • pause and resume across parts
  • cancel across parts
  • a chosen name with no-overwrite, and an unsafe name
  • headers on every connection
  • a malformed header
  • retry after a 503
  • running out of retries
  • no retry on a 404
  • cancel during a retry wait
  • pause and resume during a retry wait
• Compiled: the fixture splits a large file; a chosen name; headers plus a retry.
• Unit tests: part splitting, range headers, header parsing, which errors are transient, and the backoff.

make check passes, including clippy for x86_64-pc-windows-msvc. The two retry tests wait out the real 1 s backoff.

Checked by hand

• macOS: only through the tests above. Nothing has been run by hand in a real app yet.
• Windows: not checked. Still to do:
  • a real multi-connection download
  • pause and resume with parts
  • cancelling while several handles have the .part file open (Windows file locking)
  • a 503 retry

No backend code changed.

Follow-up

Body::dialog in codegen has the same gap the download had: each borrowed argument is held only across the next expression. hold_across covers the fix; I've left it out of this PR.