Test only small secrets in the size tests (closes #52)

The size tests for secret add, secret import and the stdin buffer no
longer store 2 MB to 101 MB secrets; the largest is 1 MiB. The cases
checking that a secret over the 100 MB limit is rejected are deleted
and not replaced. The limit itself is unchanged. With nothing large
left, the helper that skipped a case for want of locked memory is gone.

Model: opus-5-5

TODO.md

@@ -18,6 +18,12 @@ https://git.eeqj.de/sneak/secret/milestone/12
 
 # Completed Steps
 
+- 2026-10-05: No test stores a secret larger than 1 MiB
+  (https://git.eeqj.de/sneak/secret/issues/52). The size tests for `secret add`,
+  `secret import` and the stdin buffer no longer try 2 MB, 10 MB, 99 MB, 100 MB
+  or 101 MB secrets, and nothing tests that a secret over the 100 MB limit is
+  rejected; the limit itself is unchanged. With nothing large left, the size
+  tests no longer skip a case for want of locked memory.
 - 2026-10-05: The Go module path is `sneak.berlin/go/secret`, as
   `REPO_POLICIES.md` requires, not `git.eeqj.de/sneak/secret`
   (https://git.eeqj.de/sneak/secret/issues/43). Every import uses it, as do the