CI macOS 27: wrap a bootstrap mcpp that predates the float macro fix

The raw clang steps now build import std on macOS 27 (run 35142520288). The
self-host step failed because the bootstrap, 2026.9.16.1, predates the fix.
setup-macos-llvm wraps a bootstrap older than 2026.9.17.1 on macOS 27 or later
and passes the same two words through CCC_OVERRIDE_OPTIONS; the freshly built
binary runs unwrapped, and the wrapper retires with the bootstrap pin.

Author: speak-agent

.agents/docs/2026-09-17-655-implementation-plan.md

@@ -294,6 +294,18 @@ two words. Every release since #234 passed such an element as one argument, so
 the syntax gained one exception (SPEC-004 §8 rule 8): an element that begins
 with `-D` or `/D` and contains a space is one word, verbatim.
 
+The second macOS 27 run (`10224fb8`) confirmed the fix at the compiler: the raw
+clang steps print `C++23 import std works on macOS via xlings LLVM!`. It then
+failed where the job builds mcpp with the bootstrap binary, 2026.9.16.1, which
+predates the fix and so cannot build its std module on macOS 27. The
+`setup-macos-llvm` action wraps a bootstrap older than 2026.9.17.1 on macOS 27
+or later: the wrapper passes the same two words to clang through
+`CCC_OVERRIDE_OPTIONS` (quiet form, measured on clang 22.1.8: both definitions
+arrive exactly and nothing is written to stderr). The binary the job builds
+runs unwrapped, so the unit tests, the second self-host build and the e2e suite
+measure the engine's own fix; e2e 252 keeps its real old-client leg through the
+wrapper. The wrapper retires when the bootstrap pin reaches 2026.9.17.1.
+
 ## 8. Residuals
 
 - `ldflags`, `dialect_cxxflags` and `std-module-flags` keep their current

.github/actions/setup-macos-llvm/action.yml

@@ -75,6 +75,33 @@ runs:
         # `xlings install mcpp` resolved 0.0.105 here on a cold cache — below
         # the index floor, which makes every descriptor read return nothing.
         MCPP=$(bash "$GITHUB_WORKSPACE/.github/tools/install_pinned_mcpp.sh" "$GITHUB_WORKSPACE")
+
+        # A BOOTSTRAP OLDER THAN THE macOS 27 FIX, ON macOS 27, IS WRAPPED.
+        #
+        # The 27.0 SDK leaves INFINITY and NAN to <float.h> once modules are on,
+        # and mcpp states them itself from 2026.9.17.1 (hostflags.cppm,
+        # apple_float_macro_words). A released mcpp older than that cannot
+        # build its std module here, so it could not build the source that
+        # carries the fix. The wrapper hands clang the same two words through
+        # CCC_OVERRIDE_OPTIONS (a leading `#` keeps clang quiet) for the
+        # bootstrap binary only; the binary this job builds runs unwrapped, so
+        # the fix is what every later step measures. It retires itself: once the
+        # bootstrap pin is 2026.9.17.1 or later, nothing is wrapped.
+        major=$(sw_vers -productVersion | cut -d. -f1)
+        boot=$("$MCPP" --version 2>/dev/null | awk '{print $2}')
+        if [ "$major" -ge 27 ] && [ -n "$boot" ] \
+           && [ "$(printf '%s\n' "$boot" 2026.9.17.1 | sort -V | head -1)" = "$boot" ] \
+           && [ "$boot" != 2026.9.17.1 ]; then
+          wrapper="$RUNNER_TEMP/mcpp-bootstrap-macos27"
+          {
+            echo '#!/bin/sh'
+            echo "CCC_OVERRIDE_OPTIONS='# +-DINFINITY=HUGE_VALF +-DNAN=__builtin_nanf(\"0x7fc00000\")' exec \"$MCPP\" \"\$@\""
+          } > "$wrapper"
+          chmod +x "$wrapper"
+          echo "bootstrap mcpp $boot on macOS $major: wrapped ($wrapper)"
+          cat "$wrapper"
+          MCPP="$wrapper"
+        fi
         echo "MCPP=$MCPP" >> "$GITHUB_ENV"
         echo "XLINGS_BIN=$HOME/.xlings/subos/default/bin/xlings" >> "$GITHUB_ENV"