2026.10.5.2: shared-library link lines, native MSVC LTO and export discovery, std.compat on every Default row

[Sunrisepeak]

Summary

This pull request releases 2026.10.5.2. It closes #771 and #770 and the follow-ups of #768, fixes the Windows quoting of an argument that ends in a backslash, and makes a workspace's [profile.*] and its own package behave as members do. The design record is .agents/docs/2026-10-05-std-module-pair-msvc-lto-and-export-discovery-design.md (revision 6 records where the implementation departs from the reviewed revision 5). The measurements behind it are in the job summaries of the probe pull request #772.

Item	Change
D7 #771	A shared library that is not the root's links with the graph's flags (graphLdflags: the profile's ldflags, the ABI words) and the ldflags of the packages its owner reaches, in every plan. One helper builds that link group; member programs and artifact programs start from the same graph flags. The root package's private ldflags no longer reach a dependency's image; when they hold a search path or a library and the plan has a dependency's shared library, the build notes it (link/root-flags).
D8	quote_windows doubles the backslashes before a quote and at the end of an argument (the CommandLineToArgvW rules). WindowsSdkDir is read without its trailing separator. In a developer environment the llvm row's import std failed before.
D3	The COFF reader reads /bigobj objects (56-byte header, 32-bit counts, 20-byte symbols) and names /GL objects, import objects and unknown anonymous objects by their version and class GUID (measured headers).
D4/D9	lto = true on cl.exe: /GL, /LTCG on links, lib /LTCG; the summary says + lto only for what is applied.
D5	windows_auto_export keeps whether it was stated. Under cl.exe LTO, an omitted key compiles the packages linked into the DLL with /GL- and reports it once; a stated true, and /GL written into those packages' flags, are refused with the new code lto-export-discovery.
D6	mcpp pack builds the packed static libraries without LTO intermediate code (-fno-lto / /GL-).
D1/D2	Toolchain::set_std_modules / clear_std_modules are the only writers of the std / std.compat pair. std.compat is built only when a unit imports it, is provided on the GCC row (bits/std.compat.cc), and an import of it from a library without one is refused before any compile. One helper decides the .ixx source language for both std builders.
D10	[profile.<name>] is a root-position key inherited by name; a workspace root with [package] gives its own package [workspace.package] and [workspace.build] once.
D11 / O6	Glob.EscapedSpellingIsUtf8WhateverTheName no longer builds a Windows path from bytes only code page 1252 converts. [test] windows_code_page is added, and mcpp's own tests use utf-8.

src/build/prepare/plan.cpp crossed the 2500-line limit and is split at step boundaries: windows_resources.cpp and link_scope.cpp.

Closes #771. Closes #770.

Test plan

• Unit tests: argv round trip (test_windows_command_line), bigobj / /GL / import objects (test_coff_exports), the three-state key and [test] windows_code_page (test_manifest), profile and root-package inheritance (test_workspace_plan), the corrected test_modgraph.
• E2E 884 (shared-library link lines, read from RUNPATH; fails against 2026.10.5.1), 885 (workspace profiles and the root package; fails against 2026.10.5.1), 886 (std.compat on GCC and on demand; fails against 2026.10.5.1). E2E 887 (cl.exe LTO and export discovery) and 888 (the llvm row with a developer environment's WindowsSdkDir) run on the Windows shards.
• Local Linux x86_64 / gcc 16.1: full mcpp test passes (146 test programs).
• check_docs_style, check_docs_structure, check_modules_wiring, check_narrow_conversions, check_version_pins, check_file_lengths, check_workflow_assertions and git diff --check pass.
• All CI checks pass (54 checks on 29c92b5; the iOS runner probe is skipped by design).
• Self-review: the dependency-cache key now includes the /GL- or -fno-lto that scope_lto adds (29c92b5).