fix(cli): diagnose an INI overlay passed to --overlay

The INI overlay form is a SOURCE format that 'colitur convert' turns into
the S-expression one. Handing the INI to --overlay instead failed deep in
the sexp reader with 'more than one S-expression in file', which names
neither the cause nor the cure -- and the shipped examples under
data/ef/examples/ are exactly the files someone would try that with.

The detector reads past the comment header to the first real line and
looks for a [section]. Its first version capped that scan at 40 lines and
poland.ini's own header is 43, so it silently never fired on the very
file it was written for; the cap now bounds only how much of a binary
gets read, and is not a guess at header length.

1 files changed, 11 insertions, 0 deletions

diff --git a/test/cli.t b/test/cli.t
index fe18800..b2c2263 100644
--- a/test/cli.t
+++ b/test/cli.t
@@ -1270,6 +1270,17 @@ slug the engine can produce over 2020-2045:
   partial.ini: 1 of 725 celebrations named, 724 missing, 0 unknown
   partial.ini: 0 of 104 book names, 104 missing
 
+An INI overlay handed to `--overlay` is DIAGNOSED, not left to fail inside the
+sexp reader. The INI form is a source format that `colitur convert` turns into
+the S-expression one; feeding it directly otherwise produced "more than one
+S-expression in file", which names neither the cause nor the cure.
+
+  $ printf '; a comment header\n\n[overlay]\nid = x\n' > o.ini
+  $ colitur day 2027 --overlay o.ini
+  colitur: o.ini looks like an INI overlay, not an S-expression one.
+  colitur: convert it first:  colitur convert o.ini > overlay.sexp
+  [2]
+
 `--check` REJECTS an unknown slug (exit 1), so a typo is visible rather than
 silently dead -- its author would otherwise never learn why the name they
 wrote never appears: