license: relicense clectio's own code under GPL-3.0-or-later

Replace the ISC licence with GPL-3.0-or-later (Łukasz Kasprzak is sole
copyright holder, so relicensing is unencumbered):

- LICENSE: verbatim GNU GPL v3 text.
- Add "Copyright (C) 2026 Łukasz Kasprzak. SPDX-License-Identifier:
  GPL-3.0-or-later" headers to the project-authored sources (clectio.c with the
  full grant, plus mktext.c and bin2h.c), replacing their old public-domain
  notes.
- puff.c/.h keep Mark Adler's public-domain notice unchanged: third-party,
  not ours to relicense, and public domain is GPL-compatible.
- README and man page LICENSE sections updated to GPL-3.0-or-later, noting
  puff's PD status; the scripture-text carve-out (Vulgate public domain,
  corpora under their own terms) is preserved.

Code/comment change only; builds clean under -Wall -Wextra.

Claude-Session: https://claude.ai/code/session_01S1CD1u3tgSS4xNu6WHfp5a

1 files changed, 4 insertions, 2 deletions

diff --git a/clectio.1 b/clectio.1
index 1083630..348622d 100644
--- a/clectio.1
+++ b/clectio.1
@@ -150,7 +150,9 @@ Usage error (bad option, date or month).
 The full manual \(em corpus format, calendar customization, sizing \(em is the
 project README.
 .SH LICENSE
-ISC (clectio's own code). The compiled scripture text is a separate work under
-its own terms; the default Latin Vulgate is public domain.
+GPL\-3.0\-or\-later (clectio's own code; see the LICENSE file).
+.B puff.c
+is Mark Adler's public\-domain inflate. The compiled scripture text is a separate
+work under its own terms; the default Latin Vulgate is public domain.
 .SH AUTHOR
 Łukasz Kasprzak.