Clarify cache access in Handle Fetch (#1786)

The previous wording "the registration’s storage key’s name to cache map"
was ambiguous. This change clarifies the mechanism for accessing caches
during a fetch event handled by a service worker.

Specifically, it replaces the ambiguous phrase with a direct call to "obtain
a local storage bottle map" from the Storage specification, using the reserved
client's environment. This makes the process of retrieving the cache map
explicit and aligns the specification with modern storage standards.

This addresses the issue raised in #1784

Co-authored-by: Domenic Denicola <[email protected]>

Author: yoshisatoyanagisawa

docs/index.bs

@@ -3241,7 +3241,8 @@ spec: storage; urlPrefix: https://storage.spec.whatwg.org/
               1. Else if |source| is {{RouterSourceEnum/"cache"}}, or |source|["{{RouterSourceDict/cacheName}}"] [=map/exists=], then:
                   1. If |shouldSoftUpdate| is true, then [=in parallel=] run the [=Soft Update=] algorithm with |registration|.
                   1. Set |timingInfo|’s [=service worker timing info/worker cache lookup start=] to the [=coarsened shared current time=] given |useHighResPerformanceTimers|.
-                  1. [=map/For each=] |cacheName| → |cache| of the |registration|'s [=service worker registration/storage key=]'s [=name to cache map=].
+                  1. Let |caches| be the result of running [=obtain a local storage bottle map=] with |reservedClient| and "<code>caches</code>".
+                  1. [=map/For each=] |cacheName| &#x2192; |cache| of |caches|.
                       1. If |source|["{{RouterSourceDict/cacheName}}"] [=map/exists=] and |source|["{{RouterSourceDict/cacheName}}"] [=string/is=] not |cacheName|, [=continue=].
                       1. Let |requestResponses| be the result of running [=Query Cache=] with |request|, a new {{CacheQueryOptions}}, and |cache|.
                       1. If |requestResponses| is an empty [=list=], return |timingInfo|.