feat: add safe calendar import and export

Add explicit noninteractive CLI transfer modes with full event validation, idempotent duplicate handling, collision refusal, atomic import backups, and no-overwrite export creation.

Cover pure merge semantics and end-to-end rollback paths for unsupported content, invalid identities, conflicts, option combinations, and existing destinations.

README.md

@@ -6,8 +6,9 @@ inherits its colors from your terminal theme.
 
 This repository currently contains the first local-calendar vertical slice:
 date/domain logic, guarded atomic iCalendar persistence, add/edit/delete forms,
-a responsive ANSI terminal UI with month and agenda views, tests, and Linux
-launch integration. See [the product specification](docs/PRODUCT.md),
+a responsive ANSI terminal UI with month and agenda views, explicit safe
+calendar import/export, tests, and Linux launch integration. See
+[the product specification](docs/PRODUCT.md),
 [architecture](docs/ARCHITECTURE.md), and [roadmap](docs/ROADMAP.md).
 
 ## Controls
@@ -46,6 +47,28 @@ Run the tests with `meson test -C build --print-errorlogs`. Nocal reads
 positional argument. `--demo` adds a few unsaved sample appointments and
 `--print` emits a non-interactive frame.
 
+Calendar interchange is explicit, non-interactive, and network-free:
+
+```sh
+nocal --import SOURCE.ics --calendar TARGET.ics
+nocal --export DESTINATION.ics --calendar SOURCE.ics
+```
+
+Import atomically merges source events into the target. An event whose UID and
+all fields exactly match an existing event is skipped. The complete operation
+is rejected before the target is written if either file is unsafe to
+round-trip or contains empty or duplicate UIDs or invalid intervals, or if an
+imported UID collides with a different target event. A successful merge that
+adds events retains the target's exact previous bytes as `TARGET.ics.bak`; an
+import containing only exact duplicates does not rewrite the target or its
+backup.
+
+Export validates the source and atomically creates a canonical calendar at the
+destination. It refuses to overwrite any existing destination and rejects a
+source that fails event validation or that Nocal cannot safely round-trip.
+`--import` and `--export` are mutually exclusive and cannot be combined with
+`--demo`, `--print`, or `--restore-backup`.
+
 Nocal expands and safely round-trips a focused iCalendar recurrence subset:
 daily, weekly, monthly, and yearly rules with interval, count or until, common
 weekly/monthly/yearly selectors, `EXDATE`, floating times, UTC, and system IANA