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Look for a 405 neighbour only on paths the method has no route on (#2920)

When the routes for a request's method all miss its path, `#neighbours`
walks `@union` -- one greedy route per path -- for a route to answer 405
with. A 404 therefore walks two unions: its method's, then every path.

Most of that second walk cannot succeed. `collect_route_config_per_pattern`
groups routes by `pattern_regexp` and hands each group's greedy route the
pattern those routes share, so once the union for a method has missed a
path, the greedy route of every path that method has a route on has missed
it too. `compile!` now works out, per method, the greedy routes of the paths
it has no route on and compiles those into a union of their own; after a
miss, only that one is walked. In an API where every path answers GET, a GET
leaves nothing to walk.

The subset is built from the same members as `@union`, in the same order, so
a path resolves to the same greedy route either way and the 405 and its Allow
header are unchanged. A method with no routes of its own covers no path and
still walks all of `@union` -- which is also what auto-OPTIONS does, unless
OPTIONS routes are declared. Methods that leave the same paths uncovered, PUT
and DELETE on a member say, share one union. And once a route has matched and
cascaded the neighbour is never read, so it is no longer looked up.

A subset of `@union` numbers its groups differently from `@union` itself, so
each neighbour union carries the group of each of its routes rather than
reading `regexp_capture_group`.

Measured on Ruby 4.0.6 without YJIT, median of 5 interleaved rounds against an
extract of master, on a CRUD-shaped API: GET and POST on each collection, GET,
PUT and DELETE on each member, varied resource names, prefix + path version +
`format :json`. At 10 / 100 / 250 resources:

  GET 404    +74.6%  +97.9%  +93.5%
  POST 404   +24.3%  +40.4%  +31.3%
  POST 405    +5.8%  +27.7%  +21.3%

POST gains less because it has no rout... (continued)

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