Feature flags live where they are read

[Force67]

Phase 5b of docs/STRUCTURE.md.

• foundation/features/features.def, FeatureId, FeatureEnabled() and Features() are gone. A module declares a flag as a base::Feature at namespace scope in the file that reads it (base::Feature ClothFeature{"physics.cloth", true}; in physics_world.cc) and reads it as a bool. Adding a flag no longer edits a shared file.
• InitFeatures() applies RX_FEATURES by walking base::Feature's chain. Since Shared build links and runs: exports, one option registry, per-DSO volk, linux-dev #84 that chain is one per process, so a flag in rx_physics is reachable from rx_foundation in the component build too. Unknown names still warn.
• The old reason for a central table (the linker dropping an unreferenced object and its registration with it) doesn't apply when the flag sits next to its only reader: the flag is dropped exactly when the code that reads it is.
• Removed: render.pathtracer, render.mesh_shader_lod and render.distant_lod, which were declared but never read anywhere. RX_FEATURES naming them now warns instead of silently doing nothing.

Verified, static and shared:

• ctest 155/155 in both; lint 0 violations
• RX_FEATURES=-physics.cloth cloth_sim_test --feature-off logs "feature 'physics.cloth' off by RX_FEATURES" and the gate holds in both builds; a bogus name warns and leaves cloth on

Downstream: replace FeatureEnabled(FeatureId::kX) with a base::Feature declared next to its reader.