0.5.2: the island boundary runs on macOS and Windows, and a namespace segment is derived by path arithmetic

[Sunrisepeak]

This began as a coverage step and found a defect.

The coverage that was missing

mcpp.tools.island compiled on all three hosts -- tests/all-rules-compile imports it -- and had been exercised on one. What it does is path arithmetic: a namespace is the path from a root to a file, and a path separator is not the same character everywhere.

What the first run found

windows-2022, the island fixture, failing to compile:

src/app.cppm:31:21: error: no member named 'image' in namespace 'island_interface::kernels'
   31 |     return kernels::image::island_scale(a, v.data(),

namespace_of trimmed the base directory off a file's directory as a string. On Windows the two spellings differ even when the paths are the same: a caller states a root with forward slashes, directory_iterator appends with the preferred separator, so the leftover was \image rather than image. Iterating that yields the root directory as its first component, the sanitiser turns it into _, and the entry point landed in island_interface::kernels::_::image -- a namespace no consumer writes.

lexically_relative compares components, so the separator a caller happened to write is not part of the question. A base that is not a prefix now answers with no segments rather than with the whole absolute path, which is what the string form produced.

The shader lane had this too

namespace_of is shared by both lanes since 0.5.0, and rules-spirv's Windows fixture keeps every payload in one directory -- so a namespace segment was never derived there. The leg that surfaced the defect is not the lane it lives on.

A step now asserts that the derivation exists once, in the lib root: a copy reintroduced in a rule would silently take away the protection the island fixture's cross-platform run gives the shader lane.

Criteria

criterion	measured
the boundary is generated and namespaced on this host	the fixture builds, runs, and both namespace blocks are asserted -- on macOS and Windows as well as Linux
the generated header carries no namespace on any host	grep, per host
the derivation exists once	no namespace_of or common_base_dir defined outside src/plugins.cppm
Linux behaviour is unchanged	the ten island steps and the spirv module surface, including namespace default_, re-run locally

The floor does not move.