[build] Inline xxHash submodule into src-ThirdParty/

[jonathanpeppers]

Context: aligned with the ongoing external/ cleanup campaign (#11529, #11568, #11580, #11608, #11613, #11631).

We only consume the xxhash.h header from xxHash, and we use it in inline mode only (XXH_INLINE_ALL) via src/native/common/include/shared/xxhash.hh:

#define XXH_NO_STREAM
#define XXH_INLINE_ALL
#define XXH_NAMESPACE xaInternal_
#include <xxHash/xxhash.h>

No .c from upstream is ever compiled. Vendor that one header into src-ThirdParty/xxHash/ and drop the git submodule.

Changes

• Copy external/xxHash/xxhash.h verbatim to src-ThirdParty/xxHash/xxhash.h (BSD-2-Clause). Directory case is xxHash (capital H) to match the existing <xxHash/xxhash.h> include path on case-sensitive filesystems.
• src/native/CMakeLists.txt:
  • Remove the dead XXHASH_DIR variable (it was set but never referenced).
  • Add a new THIRDPARTY_DIR = ${REPO_ROOT_DIR}/src-ThirdParty variable.
• Add ${THIRDPARTY_DIR} next to ${EXTERNAL_DIR} in the six target_include_directories blocks that previously resolved <xxHash/xxhash.h> from external/:
  • src/native/CMakeLists.txt (CLR macro)
  • src/native/clr/host/CMakeLists.txt
  • src/native/clr/shared/CMakeLists.txt
  • src/native/nativeaot/host/CMakeLists.txt
  • src/native/mono/monodroid/CMakeLists.txt
  • src/native/mono/shared/CMakeLists.txt
• Remove the [submodule "external/xxHash"] block from .gitmodules.
• Remove external/xxHash from the gitsubmodule ignore list in .github/dependabot.yml.
• Update .github/skills/update-tpn/SKILL.md - move xxHash from the "Git Submodules" table to the "Vendored Source" table; reword the "Native Libraries" line to reflect the new header-only inline usage.

THIRD-PARTY-NOTICES.TXT already has a full BSD-2-Clause Cyan4973/xxHash block, so no TPN edits are required.

Why THIRDPARTY_DIR points at src-ThirdParty/ (not src-ThirdParty/xxHash/)

The include in our wrapper header is written as <xxHash/xxhash.h>, so the search root we add to target_include_directories must be the parent of xxHash/ — i.e. src-ThirdParty/ — for the xxHash/ prefix to resolve. This mirrors how ${EXTERNAL_DIR} already works today: it's external/ (parent of all submodule dirs), not external/xxHash/. The wrapper header src/native/common/include/shared/xxhash.hh is intentionally unchanged.

The variable is named generically (THIRDPARTY_DIR rather than XXHASH_DIR) so future inlinings of other external/ submodules that use the same <dir-name/header.h> include style (e.g. robin-map, constexpr-xxh3, libunwind) can reuse the same variable instead of either duplicating the ${REPO_ROOT_DIR}/src-ThirdParty literal across six target_include_directories blocks or adding yet another per-project *_DIR variable.

Verification

• git grep "external/xxHash" -> no matches.
• git grep XXHASH_DIR -> no matches.
• git submodule status -> external/xxHash is gone.
• dotnet-local.cmd build Xamarin.Android.sln builds the Mono native projects (which transitively include <shared/xxhash.hh> -> <xxHash/xxhash.h>) cleanly: 0 Errors, all surviving warnings pre-date this PR.

[Copilot]

Pull request overview

This PR removes the external/xxHash git submodule and vendors the single consumed upstream artifact (xxhash.h) into src-ThirdParty/xxHash/, updating native CMake include paths so #include <xxHash/xxhash.h> continues to resolve on case-sensitive filesystems. This fits the repo’s ongoing external/ cleanup by converting a header-only dependency from submodule → in-tree vendored source.

Changes:

• Vendor xxhash.h under src-ThirdParty/xxHash/ and rely on existing header-only inline usage (XXH_INLINE_ALL).
• Introduce THIRDPARTY_DIR in src/native/CMakeLists.txt and add it to the relevant target_include_directories blocks across CLR/Mono/NativeAOT native targets.
• Remove the external/xxHash submodule wiring and related maintenance/docs entries (.gitmodules, Dependabot ignore, TPN update skill doc).

Reviewed changes

Copilot reviewed 10 out of 11 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src-ThirdParty/xxHash/xxhash.h	Adds the vendored xxHash header (BSD-2-Clause) at the include path expected by #include <xxHash/xxhash.h>.
src/native/CMakeLists.txt	Defines THIRDPARTY_DIR and wires it into CLR include directories; removes the unused XXHASH_DIR.
src/native/clr/host/CMakeLists.txt	Adds ${THIRDPARTY_DIR} to host target include paths so xxHash/xxhash.h resolves.
src/native/clr/shared/CMakeLists.txt	Adds ${THIRDPARTY_DIR} to shared CLR target include paths.
src/native/mono/monodroid/CMakeLists.txt	Adds ${THIRDPARTY_DIR} to Mono monodroid target include paths.
src/native/mono/shared/CMakeLists.txt	Adds ${THIRDPARTY_DIR} to Mono shared target include paths (BUILD_INTERFACE).
src/native/nativeaot/host/CMakeLists.txt	Adds ${THIRDPARTY_DIR} to NativeAOT host target include paths.
.gitmodules	Removes the external/xxHash submodule entry.
.github/dependabot.yml	Drops external/xxHash from the gitsubmodule ignore list.
.github/skills/update-tpn/SKILL.md	Moves xxHash from submodules to vendored sources and clarifies header-only usage.

[jonathanpeppers]

I'm on the fence about doing this, we can discuss.