pim: personal information management, starting with a calendar

A calendar as a file tree, and tools that only know the tree:

	events/date/yyyy/mm/dd/hhmm-summary	as lived
	events/uuid/<uid>/			as stored
	ctl query alarm changed

lib/cal owns all of that. A backend supplies events and, where its
protocol allows, takes changes back -- six methods. cmd/icalfs is the
first: it reads .ics files from a directory and nothing else, because
fetching is rc/fetch's job and hget already exists. That keeps
net/http out of the binary and makes a subscribed calendar and a local
one the same thing.

The tools are rc on purpose. If the tree needs a compiled program to be
useful, the tree is the wrong shape. Three things were added to the
tree because the rc port needed them: a path from an occurrence to its
event, epoch seconds beside RFC3339, and a numeric slot for all-day
events so test(1) can compare it.

ctl reports caps, so a tool can say "read only" instead of trying and
failing. A published .ics is read only: nowhere to PUT, and no
METHOD:REQUEST to reply to. CalDAV would be read write rsvp schedule,
and that is the next backend.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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+# Go on 9front
+
+All verified on `lab.qcow2`, not inferred.
+
+## Go 1.24.x is broken on plan9/amd64
+
+Every binary dies before `main`:
+
+	M structure uses sizeclass 1792/0x700 bytes; incompatible with mutex flag mask 0x3ff
+	fatal error: runtime.m memory alignment too small for spinbit mutex
+	runtime.lockVerifyMSize() lock_spinbit.go:97
+
+The spinbit mutex landed in 1.24 and `runtime.m` falls in a sizeclass that
+cannot meet its alignment. Tested: **1.23.11 ok, 1.24.4 broken, 1.25.14 ok,
+1.27.0 ok**. The host's installed Go is 1.24.4 -- precisely the broken one --
+so `mk.sh` pins `GOTOOLCHAIN=go1.27.0`.
+
+## TLS needs a CA bundle you supply
+
+Go's x509 looks only at `/sys/lib/tls/ca.pem` on plan9, and 9front ships
+none:
+
+	SystemCertPool ERR: open /sys/lib/tls/ca.pem: file does not exist
+
+Copy any bundle there and HTTPS works -- verified TLS 1.3 with full cert
+verification. Already installed on `lab.qcow2`.
+
+## Networking is /net, not a helper program
+
+Go opens `/net/tcp/clone`, writes `connect`, and resolves through
+`/net/cs` and `/net/dns` (`net/fd_plan9.go`, `net/ipsock_plan9.go`). No
+`exec.Command`, no webfs. It is what `dial(2)` does.
+
+## No native graphics yet
+
+`9fans.net/go/draw` is a complete libdraw port and *builds* for
+`GOOS=plan9`, but at runtime it does
+
+	cmd := exec.Command(devdraw, os.Args[0], "(devdraw)")
+
+which is plan9port's helper. 9front has no `devdraw`. A native transport
+means opening `/dev/draw` and reading `/dev/mouse`, `/dev/kbd` -- plain
+file I/O, no cgo, a few hundred lines under an already-complete library.
+Nobody has written it.
+
+## Building
+
+Cross-compile from Linux; do not put a toolchain on the guest. A native
+plan9/amd64 toolchain builds fine via `bootstrap.bash` (needs a bootstrap
+Go >= 1.24.6), but it is 249MB unpacked -- `compile` alone is 27MB --
+against a 3MB `ical/fs`.
+
+Getting binaries in, with an HTTP server on the host:
+
+	hget http://10.0.2.2:8099/fs > /tmp/icalfs   # qemu user-net host
+
+## The guest clock is skewed by the host's timezone
+
+`run.sh` passes `-rtc base=localtime`, so the guest's idea of *UTC* equals
+the host's *local* time. With the host on EDT the guest is 4h behind real
+UTC. Anything time-sensitive must be generated in the guest's frame --
+take `date -n` from the guest, not from the host.