docs+test: finish the riscv64-linux-musl promotion

`mcpp toolchain list`'s sample output in docs/03-toolchains.md (+ zh) still
showed riscv64-linux-musl as `— / planned`; the real output after the promotion
is `gcc 16.1.0 / available`. Verified against a built binary, not transcribed.
The row that used to illustrate the `planned` status is replaced by
aarch64-linux-gnu so the block still shows all three states.

103_target_vocabulary.sh now reads its planned-tier target out of
`toolchain list` instead of hardcoding one. That assertion is about the TIER;
with a hardcoded triple every promotion silently turns it into a test of
something else (riscv64-linux-musl was the second target to go that way), and
the check degrades to a skip once no planned target is left.

Author: speak-agent

docs/03-toolchains.md

@@ -101,7 +101,8 @@ Targets:
   *  x86_64-linux-musl       static                gcc 16.1.0        installed
      x86_64-windows-gnu      PE, static, cross     gcc 16.1.0        installed
      aarch64-linux-musl      static, cross         gcc 16.1.0        available
-     riscv64-linux-musl      static, cross         —                 planned
+     riscv64-linux-musl      static, cross         gcc 16.1.0        available
+     aarch64-linux-gnu       cross                 —                 planned
 
 Available toolchains (run `mcpp toolchain install <family> <version>`):
      gcc 15.1.0 / 13.3.0 / 11.5.0 / 9.4.0

docs/zh/03-toolchains.md

@@ -99,7 +99,8 @@ Targets:
   *  x86_64-linux-musl       static                gcc 16.1.0        installed
      x86_64-windows-gnu      PE, static, cross     gcc 16.1.0        installed
      aarch64-linux-musl      static, cross         gcc 16.1.0        available
-     riscv64-linux-musl      static, cross         —                 planned
+     riscv64-linux-musl      static, cross         gcc 16.1.0        available
+     aarch64-linux-gnu       cross                 —                 planned
 
 Available toolchains (run `mcpp toolchain install <family> <version>`):
      gcc 15.1.0 / 13.3.0 / 11.5.0 / 9.4.0

tests/e2e/103_target_vocabulary.sh

@@ -26,11 +26,23 @@ grep -q "did you mean 'x86_64-linux-musl'" "$TMP/typo.log" || {
     echo "FAIL: missing did-you-mean suggestion"; cat "$TMP/typo.log"; exit 1; }
 
 # ── 2. planned tier: registered but unsupported → error, not a host build ───
-if "$MCPP" build --target aarch64-linux-gnu > "$TMP/planned.log" 2>&1; then
-    echo "FAIL: planned-tier target built"; cat "$TMP/planned.log"; exit 1
+# The target is read out of `toolchain list` rather than hardcoded: this
+# assertion is about the TIER, not about any particular triple, and every time a
+# target gets promoted the hardcoded spelling stops being planned and the check
+# silently becomes a test of something else (riscv64-linux-musl was the second
+# one to go that way).
+planned=$("$MCPP" toolchain list --no-color 2>/dev/null \
+          | awk '$NF == "planned" { print $1; exit }')
+if [[ -z "$planned" ]]; then
+    echo "(no planned-tier target registered — step 2 skipped)"
+else
+    if "$MCPP" build --target "$planned" > "$TMP/planned.log" 2>&1; then
+        echo "FAIL: planned-tier target '$planned' built"; cat "$TMP/planned.log"; exit 1
+    fi
+    grep -q "not yet supported" "$TMP/planned.log" || {
+        echo "FAIL: missing planned-tier error for '$planned'"
+        cat "$TMP/planned.log"; exit 1; }
 fi
-grep -q "not yet supported" "$TMP/planned.log" || {
-    echo "FAIL: missing planned-tier error"; cat "$TMP/planned.log"; exit 1; }
 
 # ── 3. escape hatch: custom triple with an explicit [target.X] section ──────
 # (pin the host gcc so resolution succeeds; the point is validation lets an