Add relationshipsOnly to select relationship routes (#13)

[evoactivity]

What

jsonApiResource could already subset resource routes with only, but relationship routes registered all-or-nothing based on whether a relationships controller was passed. This adds a separate relationshipsOnly option that lists which relationship actions to register.

router.jsonApiResource('teams', {
  resource: () => import('#controllers/teams_controller'),
  relationships: () => import('#controllers/team_relationships_controller'),
}, { relationshipsOnly: ['show', 'related'] })

Design

Two independent lists rather than one flat list:

• only selects resource routes: index, store, show, update, destroy.
• relationshipsOnly selects relationship routes: show, replace, add, remove, related.

Each token matches its controller method name. Omit a list and every route on that axis registers, so only keeps its current meaning and nothing breaks. Subsetting one axis leaves the other whole, so you do not have to enumerate every resource action just to trim relationship routes.

Changes

• src/routes.ts: new relationshipsOnly option and RelationshipActions type; split the route filter into resource and relationship checks.
• tests/unit/register_resource.spec.ts: new spec driving registerJsonApiResource with a recording fake router, covering the default set, each axis in isolation, both together, and each controller alone.
• docs/reference.md: new "Selecting routes" section with a token table per axis.
• docs/writing-data.md: pointer to the reference.

Checks

134 tests pass, typecheck clean, lint clean, formatted.

Closes #13