Add linux-firmware-cirrus stable-snapshot shim for the Dell XPS 13 DX13260

The Panther Lake XPS 13 (audio subsystem 1028:0e54) drives all of its
speakers through two CS35L56 amplifiers behind the CS42L43 codec. Their
DSP firmware aliases (cs35l56-b2-dsp1-misc-10280e54-spkid{1,2,3}) were
added to linux-firmware on 2026-08-18 and ship in Arch's
linux-firmware-cirrus 20260910-2, but the stable channel's Arch snapshot
is still on 20260810-2. Without them the amps run ROM firmware and the
machine is completely silent; upstream 7.2 already selects the sidecar
amplifier path for this SSID, so no kernel change is involved.

Ship the 20260910-2 payload to stable as a self-retiring shim:

- fast ring, no upstream watch (sync: false): the version is deliberately
  20260810-3, above the snapshot's 20260810-2 and below Arch's real
  20260910-2, so the genuine package supersedes it in the same
  transaction that upgrades linux-firmware-other once the snapshot
  advances. Bumping pkgver would defeat that.
- the signed Arch package is verified against the Arch packager key in
  keys/pgp/ and reinstalled as-is, minus the cs42l45 SDCA tree that Arch
  moved out of linux-firmware-other in 20260910: on the stable snapshot
  those 190 files are still owned by -other 20260810-2 and would
  conflict. The nine 10280e54 links and the 39 new SDCA files are kept.

Verified on a DX13260: cold boot loads 10280e54-spkid1 v4.5.9 on both
amps with "Calibration applied", and a 440 Hz tone measured through the
internal microphones peaks 238x over the noise floor on the stock UCM
bridge route. Delete this recipe once stable's snapshot carries
linux-firmware >= 20260910.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

docs/upstream-sources.md

@@ -168,6 +168,8 @@ in `origin` and has no effect on release selection.
 
 These packages were already excluded from automatic AUR updates. The migration preserves that policy.
 
+`linux-firmware-cirrus` is a deliberate hold: a self-retiring shim that ships Arch's linux-firmware-cirrus 20260910-2 payload to stable while stable's Arch snapshot is on 20260810-2 (Dell XPS 13 DX13260 / 1028:0e54 speaker firmware). It is versioned 20260810-3 so the genuine Arch package supersedes it as soon as the snapshot advances; bumping it to the Arch version would defeat that. Delete the recipe once stable's snapshot carries linux-firmware >= 20260910.
+
 ## Package-specific boundaries
 
 - NVIDIA watches remain on the 580 driver branch.