fix(wzp-video): cfg-gate dav1d + svt-av1 off Android target

shiguredo_dav1d and shiguredo_svt_av1 build scripts panic with
'unsupported target: os=android, arch=aarch64'. The AV1 SW fallback
is only needed on macOS / Linux desktop — Android uses MediaCodec
for AV1 anyway.

- Cargo.toml: AV1 SW deps moved under cfg(not(target_os = "android"))
- lib.rs: cfg-gate the dav1d and svt_av1 modules and re-exports
- factory.rs: on Android, Av1Main paths return NotInitialized when
  HW MediaCodec is also unavailable (only path on Android)
- factory tests: assert NotInitialized on Android, Ok elsewhere

Unblocks T4.3.1.1 (Android target-compile of wzp-video / mediacodec).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/wzp-video/Cargo.toml

@@ -7,11 +7,15 @@ rust-version.workspace = true
 
 [dependencies]
 bytes = { workspace = true }
-shiguredo_dav1d = "2026.1.0"
-shiguredo_svt_av1 = "2026.1.0"
 tracing = { workspace = true }
 wzp-proto = { path = "../wzp-proto" }
 
+# AV1 SW codecs do not support Android target (build.rs panics on
+# aarch64-linux-android). Android uses MediaCodec for AV1 instead.
+[target.'cfg(not(target_os = "android"))'.dependencies]
+shiguredo_dav1d = "2026.1.0"
+shiguredo_svt_av1 = "2026.1.0"
+
 [target.'cfg(target_os = "macos")'.dependencies]
 shiguredo_video_toolbox = "2026.1"