Add mapbox directions route

CHANGELOG.md

@@ -25,6 +25,34 @@ that may never merge. They are not releases and are not listed here.
   that passes them now fails with an unknown-argument error; drop the
   flags. `mapbox search forward` keeps them. (#76)
 
+- `mapbox directions`, routes between 2-25 waypoints for driving (with
+  or without live traffic), walking, or cycling. No subcommand: this API
+  has one operation, so — like `mapbox usage` — there's nothing a second
+  word would disambiguate; see `spec::FLATTENED_SERVICES`. Hand-authored
+  into `custom-openapi/` rather than waiting on an upstream spec — the whole
+  Navigation API category had no CLI coverage before this; excludes the
+  ~30 electric-vehicle-routing parameters (`engine=electric` and everything
+  under it), which describe one vehicle's charge/discharge curve down to
+  the watt and are a poor fit for a hand-typed CLI flag — left for a
+  follow-up.
+
+  The routing profile (`mapbox/driving` etc.) is a free-form value, not a
+  fixed set of four: an early version rejected anything else client-side,
+  which would have broken this command for exactly the accounts that most
+  need it, since some (OEM agreements, mainly) have additional profiles of
+  their own never published to docs.mapbox.com. Reported in review before
+  this shipped anywhere. Two path-parameter bugs surfaced while wiring the
+  original four up and are fixed for every command, not just this one: a
+  spec parameter literally named `profile` (the routing profile) silently
+  collided with the global `--profile` credentials flag, since clap has one
+  namespace of ids per command and a positional of the same name replaced it
+  outright; and a path parameter whose every legitimate value contains a
+  literal `/` (`mapbox/driving`) was being percent-encoded to `%2F` by the
+  same escaping that stops a free-text value from smuggling in extra path
+  segments — safe to skip for a parameter named in a small table
+  (`UNESCAPED_PATH_PARAMS`) as one whose values are trusted to carry that
+  character on purpose.
+
 - **Breaking**: `--exclude-fields` removed from `mapbox search forward`
   and `mapbox search category`. The `photos` and `reviews` fields it
   omitted are only returned for accounts with a specific data provider

README.md

@@ -251,6 +251,20 @@ mapbox styles <operation>
 mapbox tilesets <operation>
 ```
 
+`mapbox directions` is the one exception: its API has a single operation,
+so there's a bare command with no subcommand at all, the same shape
+`mapbox usage` already has — see [docs/commands.md](./docs/commands.md) for
+its own parameters.
+
+A command group is not the same thing as a spec file: which one an operation
+belongs to is decided per operation. So `sprites` and `tilesets` are each
+assembled from operations declared by the Styles, Raster Tiles and Vector
+Tiles specs. `mapbox tilesets` is also unrelated to `mapbox tilesets-cli`,
+which proxies to the separate Python tool.
+
+An operation can nest one level deeper where a group reads better, as in
+`mapbox styles draft get`, `draft update` and `draft delete`.
+
 For example:
 
 ```sh