feat(rsync): directory pages and edge checks in the engine

[jshvn]

Moves ctan's directory pages (pages, index) and its read-back checks into the rsync engine, behind INDEX, PAGE_FOOT and CANARY, and adds a Content-Encoding assertion on the first non-empty .tar.gz. ctan, gnu and nongnu then carry no verbs of their own. tlnet is unaffected apart from the new smoke lines.

Order: katoptra/ctan#37 (ctan excludes the engine's pages) must merge before v2.3.0 is tagged. Without it ctan fails to parse once v2 moves (checked locally: "Found multiple tasks (pages) included by rsync"); with it, ctan and tlnet both parse against this branch.

Test plan:

• examples/rsync: task check; task run -- task offline. run-pages/want is ctan's page set, drawn by ctan's pages, and the engine must match it byte for byte. smoke-refuses proves the page read-back fails without the page.
• ci workflow green.
• Live hosts send no Content-Encoding on install-tl-unx.tar.gz (checked before opening).