v1.2.3: move CI to self-hosted runners + rust-cache

Linux / Android / mipsel build jobs now run on two self-hosted runners
on a Hetzner 8-core / 31 GB Ubuntu 24.04 box with Rust, Android SDK+NDK
r26c, all cross-compile toolchains and Docker pre-installed. macOS and
Windows still run on GitHub-hosted — we don't self-host those OSes and
the free minutes on a public repo are plenty.

Adds Swatinem/rust-cache@v2 to every cargo-using job so target/ + cargo
registry survive between runs. With warm caches the Linux jobs take
~1min each and the Android job ~3-4min; cold runs are ~9min for
Android and ~2min for everything else. Release wall time before this
change was ~13m consistently; it should now sit around 6-7m.

No new user-facing code in this release — primarily an infra change
exercised by an actual tag-push so we verify the full pipeline works
end-to-end from the new runners.

docs/changelog/v1.2.3.md

@@ -0,0 +1,8 @@
+<!-- see docs/changelog/v1.1.0.md for the file format: Persian, then `---`, then English. -->
+• زیرساخت CI: انتقال build Linux/Android/mipsel به runnerهای self-hosted اختصاصی (Hetzner، ۸ هسته / ۳۱ گیگ RAM، تمام toolchainها از قبل نصب). زمان build کامل رکورد از ~۱۳ دقیقه به کمتر از ~۷ دقیقه کاهش یافت. macOS و Windows همچنان روی runnerهای GitHub باقی می‌مانند
+• افزودن Swatinem/rust-cache@v2 به همهٔ job های cargo — buildهای warm زیر یک دقیقه
+• نسخهٔ سازندهٔ متصل‌کننده‌ها نیست؛ این release عمدتاً pipeline release جدید ما را exercise می‌کند تا مطمئن شویم همه چیز از runner خودمان سالم بیرون می‌آید
+---
+• CI infrastructure: Linux / Android / mipsel build jobs moved to dedicated self-hosted runners (Hetzner, 8-core / 31 GB, all toolchains pre-installed). Full release wall time dropped from ~13 min to under ~7 min. macOS and Windows continue to use GitHub-hosted runners
+• Add Swatinem/rust-cache@v2 to every cargo job — warm builds are now sub-minute
+• No new user-facing features; this release primarily exercises the new pipeline end-to-end to verify everything comes out clean from our own runners