fix: support LLVM LTO DLL exports and optional automatic discovery

[julixian]

Summary

A release-profile MSVC-ABI DLL with LLVM LTO objects fails at coff-def with
unsupported COFF machine 0x4342. Even an annotated library reaches candidate
enumeration before its annotations can suppress the generated export list.

Export discovery now checks native COFF directives before inspecting bitcode
export intent. Any input's explicit exports still suppress automatic exports
for the entire DLL. Only unannotated inputs reach candidate enumeration. LLVM
bitcode candidates use llvm-nm from the selected compiler's installation;
the selected compiler reads IR without generating machine code. COFF and
bitcode share the symbol filter and i386 name normalization.

[targets.<name>] auto_export = false removes the discovery edge for targets
and dependency-owned shared libraries. The default stays true. The TOML and
xpkg readers implement the same boolean contract. The English and Chinese
references and lookup indexes document the key, platform scope and LTO inputs.
The new behavior is explicitly marked as unreleased source-build functionality.

Closes #762.

Criteria

Validation used a freshly built binary on Windows x64, MSVC 14.51.36231 and
Windows SDK 10.0.26100.0. The binary was built with llvm@22.1.8; the E2E
fixtures select llvm@20.1.7, which is the repository's Windows pin.

• 881_pe_auto_exports_accept_llvm_bitcode.sh failed against the fresh,
  unmodified origin/main binary at bin/automatic.def with machine 0x4342.
  It passes with this branch. It builds and loads FullLTO DLLs, runs a module
  consumer, checks DATA exports, preserves annotated surfaces, checks the
  root and dependency opt-outs, and reads ThinLTO, mixed objects and x86 input.
  The ordinary annotated COFF tests deliberately supply nonexistent LLVM
  tools in both input orders; linker-option metadata deliberately supplies
  nonexistent llvm-nm. These paths succeed without candidate enumeration.
  The script runs in the Windows E2E shards (requires: msvc python3).
• 258_shared_library_msvc_auto_def.sh passes with the fresh binary and
  native cl.exe. Native unannotated exports, annotation precedence and the
  consumer's dllimport requirement for DATA retain their existing behavior.
• The full mcpp test --toolchain llvm@22.1.8 run has 145 passing test
  executables and one failing executable. The sole failure is the existing
  Glob.EscapedSpellingIsUtf8WhateverTheName Windows code-page exception in
  unit/test_modgraph; the same filtered test fails against the unmodified
  origin/main binary. This PR does not change that code.
• New unit coverage checks IR names, comments and strings that resemble
  exports, linker metadata, DATA/COMDAT candidate types, invalid nm output,
  both bitcode signatures, i386 decoration, both manifest readers, selected
  tool argument quoting and the PE/MSVC-only planning gate. After the final
  IR-scanning optimization, all eight PeExports tests and E2E 881 pass again.
  The host-independent unit cases run on Linux, macOS and Windows CI; the
  native DLL E2E requires Windows and usable MSVC.
• LLVM 22.1.8's clang-cl.exe was also exercised directly as the IR reader:
  annotated FullLTO produces an empty EXPORTS section, and unannotated
  ThinLTO produces function and DATA exports.

Intersections

New rule or feature	Invariant it crosses	Test at the crossing
Bitcode export discovery	LTO preserves usable module code and data exports	E2E 881 A/E/G/H; PeExports.NmCandidatesKeepDataAndComdatButExcludeNonDefinitions
Intent before enumeration	Any explicit export suppresses the entire automatic surface, regardless of object order	E2E 881 B/D/F; IR intent unit cases; existing E2E 258
Per-target auto_export	Source-library dependencies retain their own target properties	E2E 881 C for root and path dependency; manifest/xpkg boolean unit cases
PE/MSVC-only opt-out	ELF, Mach-O and MinGW retain their native export behavior	PeExports.OptOutAffectsOnlyTheMsvcAbiSharedLinkForm
Selected LLVM tools	The chosen LLVM, target and argument boundaries survive Ninja emission	NinjaBackend.BitcodeExportToolsComeFromTheSelectedLlvmAndAreQuotedAsWords; E2E 881 H
IR intent parsing	Text data and debug metadata cannot change the DLL export policy	PeExports.NamesCommentsAttributesAndDataDoNotDeclareExports; linker-metadata unit cases; E2E 881 E/F

Compatibility

Existing projects keep automatic export discovery enabled. A library with
explicit exports still controls its own surface. Ordinary COFF discovery
needs no LLVM tools. Unannotated LTO inputs require llvm-nm alongside the
selected LLVM compiler; errors retain the object name and tool diagnostics.

The switch only affects MSVC-ABI PE shared links. ELF, Mach-O, MinGW, static
libraries and executables retain their export behavior. Native annotations,
link flags and explicit exports lists remain effective with discovery off.
Existing generated export commands change once when Ninja regenerates the
graph. No manifest migration is required. This is separate from #761's
same-package executable link fix and does not include that patch.

Checks before merging

The preceding revision passed platform builds and Linux/macOS E2E. Windows
E2E 881 failed before compilation because native Windows MCPP_HOME paths
were written into TOML without normalization. The shared fixture helper now
normalizes drive-absolute paths; E2E 00 covers native and mixed Windows spelling
and relative-path preservation. E2E 881 passes locally with native Windows
MCPP_HOME. The new CI run
verifies commit a702dee4; its results are pending.

• Documentation structure, style and version-pin checks pass locally.
  Move the Chinese README's CI badge outside its navigation table to match
  the English layout and correct the existing table-row mismatch.
• python .github/tools/check_workflow_assertions.py passes (22 workflows,
  zero problems).
• check_modules_wiring.sh, check_narrow_conversions.sh and
  git diff --check pass.
• No commit on the branch carries an attribution trailer:
  git log origin/main..HEAD -i --grep='Co-Authored-By' prints nothing.
• The squash merge receives an explicit subject and body; this PR is not
  merged by the contributor.