Second slice: the read path, and the fix for the bug that motivated the
rewrite.
The shell pipeline derived a filename twice. `tdl chat export` wrote the raw
Telegram name into a JSON, while `tdl dl` rendered it through a template whose
default pipes it through filenamify, which rewrites reserved characters and
collapses runs of '!'. A message whose name contained '!!' was therefore looked
up under one name and stored under another; the verifier never found it and
re-fetched it on every pass. Here a single function produces the name, and the
string it returns is used both to test for presence and to write the file, so
the two cannot disagree.
Names are stored exactly as Telegram reports them rather than reproducing
filenamify. That is a deliberate break from what the old pipeline wrote: a file
it stored under a rewritten name is not recognised and will be fetched again.
For the one chat archived so far that is a single file out of 18155, already
removed. Because names are verbatim they are not path-safe, so the code that
turns one into a path must enforce containment.
Walk yields a sequence rather than taking a callback, since the downloader
consumes a pull iterator and range-over-func converts either way without anyone
owning a goroutine. It pages newest first, where the old pipeline went oldest
first, which changes what an interrupted run leaves behind.
Message links are refused rather than guessed at, across every host Telegram
uses and the tg:// forms that carry the message id in a query parameter. A
private channel link and a public message link have the same shape, so the two
are told apart by parsing rather than by pattern.
Verified against the live chat: 18155 media messages, matching the shell
verifier, and every one of the 15548 objects already on the remote is found
under a derived name.
