Add 64 KiB head/tail and content-hash duplicate ladder (closes #61)

Replace the 1 KiB end sampling with a ladder for same-size candidates:
SHA-256 of the first and last 64 KiB, then a content hash that is the
whole file below 50 MiB (proof of identity) and gigabyte-spaced 1 MiB
samples at or above (deliberately probabilistic). Two files are
duplicates only when size, head, tail, and content all agree.

The signature gains a content column; schema bumps to version 2, so a
version 1 database is rejected and must be rescanned (unavoidable — every
stored hash changed). Because report and trees group stored signatures
across separate scans, content is computed for every shared-size file,
not only within-run head/tail collisions; size remains the sole read
gate. README "Duplicate detection" documents each rung; tests cover the
window boundaries, the 50 MiB boundary, and a multi-gigabyte sampled
case with sparse temp files.

Model: opus-4-8

TODO.md

@@ -29,6 +29,18 @@
 
 # Completed Steps
 
+- replace the 1 KiB end-window sampling with the 64 KiB head/tail plus
+  content-hash ladder (2026-09-22, branch `next`, closes
+  https://git.eeqj.de/sneak/sfdupes/issues/61): the duplicate signature
+  gains a `content` hash — the whole file below 50 MiB, gigabyte-spaced
+  1 MiB samples at or above — and the end windows widen from 1 KiB to
+  64 KiB. Schema bumps to version 2 (new `content` column); a version 1
+  database is rejected and must be rescanned, which is required anyway
+  since every stored hash changed. `report` and `trees` group by the
+  extended signature, so the ladder is applied across the whole
+  database. README "Duplicate detection" documents every rung including
+  the probabilistic large-file path.
+
 - remove the dead `files.dat` references from `Makefile`, `.gitignore`
   and `.dockerignore` (2026-09-21, branch `next`, closes
   https://git.eeqj.de/sneak/sfdupes/issues/22)