refactor(layout): unify public headers and source directories

[nick2781]

Unify public headers under include/ark/, with implementations under src/core/ and src/plugins/<name>/. Preserve <ark/...> includes, public CMake aliases, optional plugin targets, shared-library names and runtime configuration. Update source discovery, development consumers, editor exclusions and bilingual directory documentation to match.

This branch incorporates accepted main dc07079f88c099457d60ba036a517b828f50b7d6, including typed TOML manifests, launcher diagnostics, logging fixes and private Redis TLS support. All 112 moved public headers and third-party source bytes are unchanged. An exact comparison of 370 C/C++ files finds only six private include-path translations; the core interface target definition is unchanged. No runtime or lifecycle behavior is modified.

Validation on macOS at e8a6e66911136853ee53bb4a05505a9f22e3d372 (tree 531a6aa246e7406e2d6911131aeb93f021906a3f):

• Fresh Boost application, default plugins, server, examples, Protobuf, native tests and benchmark build; complete core, manifest/launcher, XML, kernel, network and private database regression targets passed.
• Fresh Standalone Asio core/private database build, applicable tests and benchmark self-test passed.
• Real XML and TOML application startup followed by orderly SIGTERM shutdown passed.
• clang-format 18, 34 CI/README policy tests, six Nimbus content tests and complete site/type/link checks passed.
• Independent source and integration audits confirmed the moved headers, vendor files and accepted main changes are preserved.

Hosted Windows/Linux checks must pass on this updated head before queue acceptance. Optional PhysX/legacy database plugin builds and live database servers were not exercised in this layout validation. Public database plugin activation, shutdown policy and hot updates are outside this PR.

Scheduling: prioritize this older layout PR before newer documentation PRs #66/#69 and workflow PR #70. They do not block this migration.