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

TODO.md is the project's own task list, judged by the same rules as
README.md: short sentences, lists as bullets, nothing a newcomer reads
twice. Every completed step is kept with its date and its issue or
branch, and every fact the old entries recorded is kept too, in short
sentences. The 2026-07-22 policy-compliance checklist is folded into
the step that did the work, and the accepted divergence from the
layout policy gets a section of its own. Workflow, Status and Next
Step keep their rules unchanged.

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)
 
-- The hash phase gives three hashes to every file that the walk records. `head`
+- Every file the walk records gets three hashes in the hash phase. `head` and
-  and `tail` are the SHA-256 of its first and last 64 KiB. `content` is the
+  `tail` are the SHA-256 of its first and last 64 KiB. `content` is the SHA-256
-  SHA-256 of the whole file under 100 MiB, and of 1 MiB at each gigabyte from
+  of the whole file under 100 MiB, and of 1 MiB at each gigabyte from 100 MiB
-  100 MiB up. Gone are:
+  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
@@ -231,8 +231,7 @@ 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 that the phase starts. It
+  five, as `[2/5] walk`. It runs from the moment the phase starts and shows:
-  shows:
     - the progress;
     - the elapsed time;
     - the ETA, the time remaining.