feat: standalone calendar overrides (calendar.h) + document customization

Add a compile-time override table for MINOR local calendar changes, standalone
(no lectio, no toolchain): edit calendar.h (copied from calendar.def.h), add a
{ month, day, colour, "name" } row, make. It replaces a fixed date's name and
colour, every year, in both single-day and --month output. Limits (no engine):
fixed dates only; name/colour only, not readings or rank/precedence; an override
wins unconditionally.

README gains a "Customize the calendar" section documenting both tiers with
their limits: the standalone override above, and -- for readings, ranks and
movable feasts -- regenerating gen/ from a lectio calendar layer via
clectio-gen (-caldir/-use), then rebuilding with CORPUS= to repack any newly
cited verses, with a git-checkout revert note.

Both forms build clean under -Wall -Wextra; binary unchanged at 666 KB; default
(empty override table) output is unchanged.

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

1 files changed, 25 insertions, 0 deletions

diff --git a/calendar.def.h b/calendar.def.h
new file mode 100644
index 0000000..3a43f0a
--- /dev/null
+++ b/calendar.def.h
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+/* clectio local calendar overrides -- suckless style: copy to calendar.h,
+ * edit, `make`. Compiled in; there is no config file at run time.
+ *
+ * Each row REPLACES the name and colour of ONE fixed date (month, day), every
+ * year. This is for MINOR local changes only. clectio has no calendar engine,
+ * so it CANNOT, here:
+ *   - change a day's READINGS, or its RANK / precedence (it stores neither);
+ *   - place MOVABLE feasts (Easter-relative dates);
+ *   - arbitrate against the universal day -- an override wins UNCONDITIONALLY,
+ *     so only override a date whose celebration you truly mean to replace.
+ * The readings shown on an overridden day stay the underlying day's -- which is
+ * correct for a memorial (it uses the weekday readings). For proper readings,
+ * ranks, or movable feasts, regenerate the calendar from lectio (see README).
+ *
+ * Colours: GREEN  WHITE  RED  VIOLET  ROSE  BLACK
+ * Fields:  { month, day, colour, "Display name" }
+ */
+static const Override overrides[] = {
+	/* Uncomment, edit, or add rows. Examples: */
+	/* { 5, 16, RED,   "Saint Andrew Bobola, priest and martyr" }, */
+	/* { 4, 23, RED,   "Saint Adalbert, bishop and martyr" },      */
+	/* { 8, 15, WHITE, "The Assumption -- Our Lady of the Herbs" }, */
+
+	{ 0, 0, 0, 0 } /* end marker -- keep this row last */
+};