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

0 files changed, 0 insertions, 0 deletions