[DO NOT MERGE] probe macOS 27 runner labels - #658

GitHub serves macOS 27 under the preview label xcode-27 (macOS 27.0 with Xcode
27, actions/runner-images#14404); there is no macos-27 label. Measured on #658:
the label delivers macOS 27.0 (26A5406e), arm64, ImageOS macos27.

- ci-macos and ci-macos-e2e run on macos-15 and xcode-27.
- ci-fresh-install runs the xlings and Homebrew channels on macos-14 (the
  floor) and xcode-27 (the newest release).
- Every macOS 27 leg asserts sw_vers major 27, because the label names an
  Xcode and its base OS has changed once.
- setup-macos-llvm takes the image as an input and puts it in the xlings cache
  key, which named macos15 for every image.

…mpiler receives (#655); macOS 27 (#657)

* One reading of a compile-flag element; the databases list what the compiler receives (#655)

An element of `cflags`, `cxxflags` and `asmflags` now has one meaning on every
host: the POSIX shell's word syntax without expansions, where a backslash
escapes only a blank, a quote or a backslash (mcpp.manifest.flag_words,
SPEC-004 §8). The ninja writer quotes each word for its host, and
compile_commands.json and `mcpp emit build-database` list the words, so the
databases equal the build by construction instead of re-reading command lines.

Before, an element meant whatever the host's command-line reader made of it
(measured on Linux and Windows runners, #656: five of fourteen spellings gave
different macros), and the databases listed the text with a third reading,
which is how libarchive's `-DPLATFORM_CONFIG_H=\"...\"` reached consumers with
its backslashes.

- A `defines` entry is one value: `N="x"` reaches the compiler as `-DN="x"`
  (it arrived as `-DN=x`). Features, globs, targets and `mcpp:cfg=` alike.
- Link units keep their rendered text; GAS units' database entries list their
  edge's list; the `-std` guard, dialect promotion and include normalisation
  read words.
- Windows `shell_quote_arg` doubles backslashes before a quote and the closing
  quote.
- The first plan after an upgrade names an element whose words changed
  (`build/flag-words`); a repeated plan says nothing. No flag-list element of
  the published index changes.

Tests: FlagWords.* (syntax table and round trip), CompileCommandsArgs.*
(edge equals database, host quoting round trip, /bin/sh as the reference),
e2e 736 (same assertions on every host; fails on 2026.9.16.2).
Records: .agents/docs/2026-09-17-655-*.
Version 2026.9.17.1.

* 655: the ecosystem verification script

* CI on macOS 27: ci-macos, ci-macos-e2e and both fresh-install channels

GitHub serves macOS 27 under the preview label xcode-27 (macOS 27.0 with Xcode
27, actions/runner-images#14404); there is no macos-27 label. Measured on #658:
the label delivers macOS 27.0 (26A5406e), arm64, ImageOS macos27.

- ci-macos and ci-macos-e2e run on macos-15 and xcode-27.
- ci-fresh-install runs the xlings and Homebrew channels on macos-14 (the
  floor) and xcode-27 (the newest release).
- Every macOS 27 leg asserts sw_vers major 27, because the label names an
  Xcode and its base OS has changed once.
- setup-macos-llvm takes the image as an input and puts it in the xlings cache
  key, which named macos15 for every image.

* macOS 27: the std module builds; the -D compatibility rule; host-shaped test fixes

macOS 27 (measured on the xcode-27 image, #659): the 27.0 SDK's math.h leaves
INFINITY and NAN to <float.h> once modules are on, and clang 22's header does
not supply them under -std=c++23, so libc++'s std module stopped building.
apple_float_macro_words (hostflags.cppm) states both with the SDK's own GNU-mode
spellings for clang on an Apple target; its readers quote the words their own
way (ninja text, the std module command, graph std flags, build program argv).
gnu++23, -U__STRICT_ANSI__ and -D__need_infinity_nan were rejected for their
effect beyond the defect. The raw clang steps of ci-macos state the same words.

Compatibility: an element beginning with -D or /D that contains a space is one
word, verbatim, as every release since #234 passed it (SPEC-004 §8 rule 8).

Windows: flag_element no longer quotes a backslash the syntax does not read as
an escape, normalize_include_flags keeps the spelling of an unchanged element,
and the split_flags quoting tests follow the host's reader.

* 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.

* Review: the diagnostic block sits above the defines comment; correct the 252 claim; index reading with the -D exception

- report_flag_words_changes was inserted between fold_build_defines_into_flags
  and its comment; it now precedes the comment.
- The record claimed e2e 252 keeps its real old-client leg on macOS through the
  wrapper. It does not, on any macOS leg including main: MCPP_BOOT is the xlings
  shim. Corrected and listed as a residual, with dialect promotion reading words.
- The index reading script applies the -D exception: 285 literals, 0 differ.

---------

Co-authored-by: speak-agent <248744407+speak-agent@users.noreply.github.com>