S4: catalog discovery — .relix/ directories up the hierarchy, nearest-wins overlay #8

Part of #17. Design: docs/design.md §5.1–§5.2.

Scope

• Discovery: walk from the working directory (or -C) upward, collecting .relix/ directories.
  • Stop after $HOME, the same ceiling profiles.json is found under.
  • A .relix/root file stops the walk at that directory.
  • Outside $HOME, only the working directory is read.
• Load order, outermost first:
  1. ~/.relix/catalog/*.relix
  2. each project level's catalog/*.relix, in lexical order within a directory
  3. --catalog=FILE and RELIX_CATALOG_PATH (:-separated)
  4. -i bindings
  5. the script
• Overlay: a nearer declaration replaces a farther one of the same name. Each file is parsed with Relix.parse(text, source), the nearest declaration of each name is kept, and the survivors are defined with define(Statement...).
• relixrc: key = value defaults (output, profile), nearest wins, with command-line options beating it.
• -N/--no-catalog turns discovery off.
• Declaring opens nothing: no connection or introspection happens until a query references the name.

Dependencies

• S1 (S1: command skeleton — script sources, exit codes, diagnostics, profiles, engine options #4).
• A relative source path in a catalog file resolving against that file's directory is Resolve a source's relative path against the file that declares it relix-core#81. Until it lands, catalog files must use absolute paths or placeholders, and that is documented as a limitation.
• Untrusted project levels are skipped once S5 (S5: catalog trust — direnv-style trust for project .relix/ directories #9) lands. Until then this slice must not be released on its own.

Acceptance

• @TempDir trees cover:
  • the $HOME ceiling, the root marker and the outside-$HOME case
  • lexical order
  • nearest-wins, including an outer view reading an inner replacement of its source
  • -N, --catalog and RELIX_CATALOG_PATH
• A piped script (cat q.relix | relix) resolves catalog names.