033-configured-corpus-root

The coupling gate honors `layout.specs_dir`

`layout.specs_dir` is configuration, and every other consumer reads it: `compile` scans it, `index` builds each spec's `spec.md` path from it, and `init` scaffolds into it. The coupling gate did not. Two functions in `couple.rs` hardcoded the literal `specs/`, and both are load-bearing. The primary-owner heuristic asked whether `specs/<id>/spec.md` was in the diff, so for an adopter whose corpus lives anywhere else the answer was permanently no: no edit to an owning spec could clear `C-001`, and every governed change was refused with the owner named and no reachable way to satisfy it, short of a waiver on every PR. The amends-awareness parser had the same literal, so amendment expansion silently never fired. Both now take the configured root, and the builder is shared with the indexer (`spec_md_rel`, promoted to `pub(crate)`) so a path constructor and its parser cannot drift apart again. A trailing slash on the configured value is trimmed, making `specs` and `specs/` name one corpus root instead of the second yielding `specs//<id>/spec.md`. No DTO, schema, or config shape changes; a default-layout repo is byte-identical.