Rewrite TODO.md sentence by sentence (closes #135)

Rewrites `TODO.md` from scratch for
#135, by the rules applied to
`README.md` in #140. It is the
project's own task list, so every completed step keeps its date, its
issue or branch and every fact that the old entry recorded, in short
sentences, with lists as bullets. An entry superseded later says so in
one sentence.

`TODO.md` records history, so its entries were not corrected against the
code. The 2026-10-17 release-rule entry is known to be stale. One product
name in the 2026-10-15 entry is replaced by a description.

This file carries the `TODO.md` entry of
#140 word for word, so the
conflict once that merges is resolved by taking this file whole.

Model: fable-5-1

TODO.md

@@ -63,10 +63,10 @@ are known.
   `trees`. Reading a record whose hash is not 32 bytes is an error (2026-10-26,
   https://git.eeqj.de/sneak/sfdupes/issues/134)
 
-- Every file the walk records gets three hashes in the hash phase. `head` and
-  `tail` are the SHA-256 of its first and last 64 KiB. `content` is the SHA-256
-  of the whole file under 100 MiB, and of 1 MiB at each gigabyte from 100 MiB
-  up. Gone are:
+- The hash phase gives three hashes to every file that the walk records. `head`
+  and `tail` are the SHA-256 of its first and last 64 KiB. `content` is the
+  SHA-256 of the whole file under 100 MiB, and of 1 MiB at each gigabyte from
+  100 MiB up. Gone are:
     - the 10 MiB and 50 MiB thresholds;
     - the size census, which counted the sizes of every walked file and of every
       record outside the operands. A file was hashed only when the census showed
@@ -231,7 +231,8 @@ are known.
     on a slow reader (2026-10-16, https://git.eeqj.de/sneak/sfdupes/issues/113)
 
 - Each `scan` phase's progress display names the phase and its place among the
-  five, as `[2/5] walk`. It runs from the moment the phase starts and shows:
+  five, as `[2/5] walk`. It runs from the moment that the phase starts. It
+  shows:
     - the progress;
     - the elapsed time;
     - the ETA, the time remaining.