docs: require abstract example values, and fix a stale sample

Real values from a live run had reached the README, the demo and several
fixtures: a chat name, its dialog id, Telegram file ids, filenames from the
chat, and the archive's own statistics. This repository archives a private
chat, so those name a real account and its contents, and history keeps them
long after the file is edited.

The values are gone from every commit. CLAUDE.md records the rule so they do
not come back, with a placeholder for each kind — a fixture keeps the property
it exists to test, so a multi-byte filename test still gets a multi-byte name,
just an invented one.

README.md

@@ -78,8 +78,8 @@ indexing PikPak root 'mychannel'
 
   ↓ total    26/606 files [=>             ] 617.5 MiB / 79.0 GiB  2.5 MiB/s  8h47m
   ↑ total    24/606 files [=>             ] 598.0 MiB / 79.0 GiB  2.4 MiB/s  8h58m
-  ↓ …3214_4242_1000000000000000001.mp4   [=======>       ]  41.2 MiB / 96.0 MiB  1.8 MiB/s
-  ↑ …3214_4243_1000000000000000002.mp4   ⠹                         uploading 1.9 GiB
+  ↓ …7890_4242_1000000000000000001.mp4   [=======>       ]  41.2 MiB / 96.0 MiB  1.8 MiB/s
+  ↑ …7890_4243_1000000000000000002.mp4   ⠹                         uploading 1.9 GiB
 ```
 
 The two legs are counted separately because they run at different speeds and