015-short-id-resolution

Compile: resolve short spec ids on `depends_on` and `superseded_by`

Lets a `depends_on` or `superseded_by` reference name its target by the leading spec number alone (`109`) and have the compiler resolve it to the full id (`109-slug`) when exactly one spec matches that number, rewriting the reference before validation and emission. An exact id, an ambiguous number, or a number matching nothing is left unchanged, so V-008 (superseded_by must resolve) and V-010 (dangling depends_on) still fire on a genuinely broken reference. The resolution policy is the indexer's existing `resolve_id` (spec 004), applied at compile time so the registry and the index agree. Unblocks adopting a corpus that references specs by number (106 specs in the OAP dry-run) without a mass frontmatter migration; a no-op on any corpus that already uses full ids (such as this repo's own).