feat(cli): --overlay FILE, so a local calendar can be supplied

The Overlay algebra has been built and tested since Plan 2, and
Date_spec grew movable variants last commit, but there was still no way
to hand colitur a file: bin/main.ml applied exactly one overlay, the
shipped data/ef/adjustments.sexp, with the path hardcoded. This is the
plumbing.

--overlay is repeatable and ordered, and applies ON TOP of the shipped
adjustments rather than instead of them. That ordering is the whole
point: adjustments.sexp carries RG 110's own 30 June companion, the Major
Litanies, St Barbara and Rogation Wednesday, and a user file that
replaced it would silently drop all four while appearing merely to add a
local feast. Overlay.merge is last-writer-wins, so a local calendar can
still override a universal entry deliberately, by naming its slug.

The dispatch needed real argument parsing. It matched Sys.argv as an
exact array, which does not survive a repeatable flag -- two --overlay
arguments are a different array shape from one, and each further flag
would multiply the patterns again. Flags are now stripped first and the
remaining words matched as command plus year. Hand-rolled, because the
dependency list is frozen and this is fifteen lines. One defect found by
the cram suite on the first run: the unknown-option guard rejected
--help and --version, having excluded only their short forms.

Refused, not ignored, on easter and temporal. Neither reads sanctoral
data -- temporal deliberately runs the cycle before any layer exists --
so accepting the flag there and silently doing nothing is the failure
mode this project refuses everywhere else.

A file that fails to load is fatal, exactly as the shipped overlay is; a
directive naming a slug that does not exist warns on stderr and the run
continues. The second matters more for a user file than for ours: a typo
in a diocesan calendar should say so rather than quietly do nothing.

test/fixtures/overlay-example-diocesan.sexp is a worked example and is
labelled as invented, not calendar data. It carries both shapes a real
local calendar needs: a fixed-date patron, and a dedication on "the first
Sunday of October" -- which is the case the Date_spec work existed to
unlock and which lands on 4 October in 2026, 3 October in 2027, 1 October
in 2028. The dedication is I class because a church's own dedication
anniversary is I class in that church; at III class it lost to the
II-class Sunday every year, which a first draft demonstrated correctly
and uselessly.

The man page gains an OVERLAYS section saying plainly that an overlay is
applied, NOT validated: the five test layers assert things about the
shipped calendar and none of them can vouch for a file the user supplies.
This is the first feature that lets someone change what colitur computes,
and that distinction should not have to be inferred.

1 files changed, 15 insertions, 3 deletions

diff --git a/CLAUDE.md b/CLAUDE.md
index f09c555..8edcdb2 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -71,8 +71,8 @@ overlays**.
   year into a rata-die table, with the domain filter in `Layer.index` itself
   (both edges, 1582 and 10000, bit during development). This unblocked
   **Rogation Wednesday** (RG 87/88/89, register §6.10) and is what a
-  user-supplied overlay needs for a local movable feast. The `--overlay` CLI
-  plumbing is **not yet built**.
+  user-supplied overlay needs for a local movable feast. The `--overlay` CLI plumbing
+  is **built** (see below).
 - **Kernel** (`lib/kernel`, pure, total, bounded 1583–9999): `Computus` (Gregorian
   + Julian Easter + anchors), `Date` (proleptic Gregorian arithmetic), `Overlay`
   (ordered layer-merge algebra: field-level add/suppress/replace/edit, last-writer-
@@ -303,10 +303,22 @@ M20 added by the `ef-major-litanies` task, M18 394 not 395 accordingly).
 Fixtures live in `test/fixtures/` with asserted SHA-256s.
 
 **CLI**: `colitur easter <year>`, `temporal <year>`, `day <year>`,
-`readings <year>`, `-h`/`--help`, `-V`/`--version`. Man page in
+`readings <year>`, `-h`/`--help`, `-V`/`--version`, and `--overlay FILE`
+(repeatable, ordered; `day`/`readings` only). Man page in
 `man/colitur.1`; `Makefile` installs binary + data + man page into `~/.local`
 by default. Tagged **v0.1.0**.
 
+**User overlays** apply ON TOP of the shipped `adjustments.sexp`, never
+instead of it — replacing would silently drop RG 110's companion, the Major
+Litanies, Barbara and Rogation Wednesday. `Overlay.merge` is
+last-writer-wins, so a local calendar can still override a universal entry
+by naming its slug. Refused on `easter`/`temporal` (they read no sanctoral
+data) rather than silently ignored. **An overlay is applied, not validated**
+— the five layers assert things about the SHIPPED data and cannot vouch for
+a user file; a directive matching nothing warns on stderr and continues, a
+file that fails to load is fatal. Worked example, both a fixed and a movable
+local feast: `test/fixtures/overlay-example-diocesan.sexp`.
+
 `--help` prints to **stdout** and exits **0**; a usage error prints one line
 to **stderr** and exits **2**. The distinction is asserted in `test/cli.t`,
 both directions, because it is the sort of thing that silently rots.