pnpm/rfcs#16 was updated to rename the config surface: every explanation
of a mount began "a mount is a full npm registry at...", so the unit is
now simply a **registry**. `mounts:` becomes `registries:` and
`defaultTarget:` becomes `defaultRegistry:` (npm's own concept), aligning
the server with the client-side `namedRegistries`. pnpr is pre-1.0, so
the old keys are replaced outright with no compatibility mode.

The Rust surface follows the RFC's reference sketch: `MountKind` →
`Registry`, `Mounts` → `Registries`, `MountConfigError` →
`RegistryConfigError`, the `mount` module → `registry`, `Config.mounts`
→ `Config.registries`, and prose/error messages now say "registry"
("name a hosted registry", `/~<name>/`). The axum-level "routes are
mounted" wording is kept — that is framework vocabulary, not the
renamed concept. The vocabulary is now: **registry** — a named surface
pnpr serves; **origin** — the external URL an upstream registry fetches
from; "uplink" remains only as the internal upstream-backend term.

… sources (#12778)

Implements the revised model from pnpm/rfcs#16, replacing the
route-level-pattern shape outright (pre-1.0, no compatibility mode).

The config surface is renamed per the RFC: `mounts:` -> `registries:`,
`defaultTarget:` -> `defaultRegistry:`, and the Rust types follow
(`MountKind` -> `Registry`, `Mounts` -> `Registries`, the `mount` module
-> `registry`). The vocabulary is now: registry — a named surface pnpr
serves; origin — the external URL an upstream registry fetches from.

Hosted and upstream registries take an optional `patterns:` list — their
declared namespace (omitted = every name) — and a router collapses to an
ordered `sources:` list: a package resolves to the first listed source
whose patterns claim it. The namespace is enforced at the registry, on
every path to it: an off-pattern read is a definitive 404 answered before
storage or the upstream is consulted, and an off-pattern publish is
rejected with a clear reason — through a router and at the registry's own
`/~<name>/` URL alike. This closes the open hosted namespace (no dormant
stored state that a later source edit would surface as authoritative) and
stops an authorized caller from pulling arbitrary public names through a
private upstream's server-owned credential.

Validation translates to the same `covers()` machinery: unreachable
sources (all claims covered by earlier sources' union, including a
non-last pattern-less source), per-pattern shadowing across sources
(identical claims by two sources rejected, so bidirectionally-overlapping
namespaces fail in either order), duplicate sources per router, duplicate
patterns per registry, plus the carried-over unknown/self-referential/
router-as-source/empty-router checks. A registry's own internally
redundant patterns are allowed and do not count as self-shadowing.

Patterns live only in the registry graph (`Config::registries`), not
duplicated into the hosted/uplink tables: enforcement happens in
`Registries::resolve`, which every read, write, search, and cache-header
decision flows through, so the namespace is one declaration. The bundled
config.yaml moves the registry-mock fixture list onto the `local`
registry, and `Config::proxy` / `Config::static_serve` build the
equivalent graphs programmatically.