refactor(manifest): split into :types / :toml / :xpkg partitions

manifest.cppm mixed three concerns in 2560 lines: the shared data model,
mcpp.toml parsing, and the xpkg .lua mcpp-segment reader. Now a directory
module with one data model and two independent surface grammars:

  src/manifest/manifest.cppm  primary — export import :types/:toml/:xpkg
  src/manifest/types.cppm     Manifest/Target/BuildConfig/errors + shared
                              helpers (module linkage, not exported)
  src/manifest/toml.cppm      parse_string/load/default_template
  src/manifest/xpkg.cppm      LuaCursor, identity, synthesize_from_xpkg_lua

Module name and exported API unchanged — zero importer churn. Full test
suite + e2e 93 green.

Author: Sunrisepeak