fix: name Features install layers by sanitized identity

[wcgomes]

Summary

Features install layers are now named by sanitized identity (feature-<slug>) instead of sequential feature-N.

• Name Features install layers by sanitized identity so layer names stay stable across Features order changes.
• Bump recipeVersion 6→7 to invalidate cached recipes that used the old feature-N names.
• Linux tests skip Darwin PTY proofs and fix inherited-stdio + traversal fixture.
• Wiki epoch/path updates; README tests badge bumped to 808+.

Commits

• 1a5f444 fix: name Features install layers by sanitized identity
• 4809380 test: cover feature slugs and skip Darwin PTY proofs on Linux
• 8ee8855 docs: record recipeVersion 7 and bump tests badge to 808+