fix(build): drop compile-time x86 target-features — runtime AES dispatch

Users on CPUs without AES-NI (Nehalem i7-860, pre-2010 Intel Macs) or
without SSSE3 (AMD Phenom II) crashed at launch with SIGILL. The
global `-C target-feature=+aes,+ssse3` was kept after the #353/#343
x86-64-v3 incident on the belief that the aes crate's cpufeatures
runtime check would protect CPUs without AES-NI — but a compile-time
target feature makes cpufeatures compile its check to a constant, so
the AES-NI intrinsics ran unconditionally (credentials AES-GCM decrypt
at startup), and +ssse3 let LLVM autovectorize anywhere, breaking
Phenom II. Measured on the qbz-cmaf hot path (Ctr128BE<Aes128>
apply_keystream, 256 MB, release): 2145 MB/s compiled-in vs 2162 MB/s
runtime-dispatched, identical output — the flags bought nothing on
capable CPUs. Removed from .cargo/config.toml, all dev/build scripts
and the release/build workflows; old CPUs now get the software AES
fallback instead of a crash.

Fixes #549

cc

scripts/slint-run.sh

@@ -51,9 +51,10 @@ else
   echo "[slint-run] WARNING: only ${avail_mb} MB free — lowest-memory tier (slow). Close apps / drop to a TTY for a faster build." >&2
 fi
 
-# RUSTFLAGS OVERRIDES (does not merge) the .cargo/config.toml rustflags, so the
-# AES-NI/SSSE3 features must be re-listed (keep in sync with slint-dev.sh).
-export RUSTFLAGS="-C target-feature=+aes,+ssse3 -C link-arg=-fuse-ld=mold -Z threads=${THREADS}"
+# No x86 target-features here or in .cargo/config.toml (#549): the aes crate
+# runtime-dispatches to AES-NI at identical speed, and compile-time features
+# SIGILL older CPUs.
+export RUSTFLAGS="-C link-arg=-fuse-ld=mold -Z threads=${THREADS}"
 export CARGO_PROFILE_RELEASE_CODEGEN_UNITS="${CODEGEN_UNITS}"
 export CARGO_PROFILE_RELEASE_OPT_LEVEL="${OPT}"
 export CARGO_BUILD_JOBS="${CARGO_BUILD_JOBS:-1}"   # one rustc at a time (memory)