fix: restore the project's toolchain default, and stop assuming it is installed

mcpp.toml's `[toolchain] default` went from gcc@16.1.0 to llvm@22.1.8 in the
previous commit. Not deliberate -- a local experiment wrote it and `git add -A`
carried it along. The whole repository then built with llvm, and five CI jobs
failed on assertions that name gcc:

  toolchain: gcc (cold self-host)     grep -q "Resolved gcc@16.1.0"
  toolchain: musl + llvm              sed expects `default = "gcc@16.1.0"`
  integration / e2e 1-2 (linux)       `gcc@16.1.0 is not installed`

Verified back to exactly one difference from main: the version.

The e2e job's failure had a second cause worth fixing on its own. It runs
`toolchain default gcc@16.1.0` without ever installing gcc, so it depends on
the sandbox cache happening to carry one from an earlier run. That is not a
property any job should rely on, and this branch's xlings pin bump moved the
whole cache-key prefix, so every shard started cold and hit it. Now installed
explicitly; warm runs pay a lookup.

  67 unit tests; e2e 201/86/29/103/179/200 under the restored gcc default.

Author: speak-agent

.github/workflows/ci-linux-e2e.yml

@@ -78,6 +78,16 @@ jobs:
           "$MCPP" self config
           # Pin the global default so test 28 (default-toolchain path) gets a
           # deterministic GNU answer instead of an auto-install pick.
+          #
+          # Installed explicitly, not assumed. `toolchain default` requires the
+          # toolchain to be present, and nothing in this job puts it there:
+          # building mcpp above uses whatever first-run picks (llvm). It worked
+          # only because the sandbox cache happened to carry a gcc some earlier
+          # run had installed -- so the moment the cache key changed (an xlings
+          # pin bump moves the whole prefix), every e2e shard failed with
+          # `gcc@16.1.0 is not installed` before running a single test.
+          # Warm runs re-install nothing; this costs a lookup.
+          "$MCPP" toolchain install gcc 16.1.0
           "$MCPP" toolchain default gcc@16.1.0
           # Warm musl once so fresh-home e2e tests inherit the payload.
           "$MCPP" toolchain install gcc 16.1.0-musl

mcpp.toml

@@ -17,7 +17,7 @@ default-profile = "release"
 include_dirs = ["src/libs/json"]
 
 [toolchain]
-default = "llvm@22.1.8"
+default = "gcc@16.1.0"
 macos   = "llvm@22.1.8"
 windows = "llvm@20.1.7"