C12.0: ElyMail backend moves to Rust crates; iCloud by app password

async-imap, async-pop, mail-send, mail-parser and mail-builder replace
MailCore 2. The MailCore 2 and libetpan fetch, build and probe scripts
go. HTML bodies render as a sanitized block subset in GPUI; compose uses
the Markdown document component. OAuth client IDs come from oauth.env
(ignored) or CI secrets. The task list for C12 is rewritten in order.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

scripts/build-libetpan.sh

@@ -1,42 +0,0 @@
-#!/usr/bin/env bash
-# Builds the pinned libetpan into engines/libetpan.
-set -euo pipefail
-
-root=$(cd "$(dirname "$0")/.." && pwd)
-src=$root/reference/libetpan
-prefix=$root/engines/libetpan
-work=$root/engines/libetpan-src
-pin=$(git -C "$src" rev-parse HEAD)
-
-if [ "$(cat "$prefix/PIN" 2>/dev/null)" = "$pin" ] && [ -f "$prefix/lib/libetpan.a" ] &&
-  grep -q "^#define HAVE_OPENSSL 1" "$prefix/BUILD-config.h" 2>/dev/null &&
-  grep -q "^#define HAVE_JSON 1" "$prefix/BUILD-config.h" 2>/dev/null; then
-  echo "ok    libetpan $pin"
-  exit 0
-fi
-
-ssl=${OPENSSL_DIR:-$(brew --prefix openssl@3)} || {
-  echo "build-libetpan.sh: set OPENSSL_DIR to an OpenSSL 3 prefix" >&2
-  exit 1
-}
-[ -f "$ssl/include/openssl/ssl.h" ] || {
-  echo "build-libetpan.sh: no OpenSSL headers under $ssl" >&2
-  exit 1
-}
-jobs=$(sysctl -n hw.ncpu 2>/dev/null || nproc)
-rm -rf "$work" "$prefix"
-rsync -a --exclude .git "$src/" "$work/"
-cd "$work"
-./autogen.sh --prefix="$prefix" --enable-static --disable-shared \
-  --with-openssl="$ssl" --with-json --disable-db
-for feature in HAVE_OPENSSL HAVE_JSON; do
-  grep -q "^#define $feature 1" config.h || {
-    echo "build-libetpan.sh: configure left out $feature" >&2
-    exit 1
-  }
-done
-make -j"$jobs"
-make install
-cp config.h "$prefix/BUILD-config.h"
-echo "$pin" >"$prefix/PIN"
-echo "built libetpan $pin"