fix: delete $::{'Pkg::'} now properly removes namespace symbols

[fglock]

Summary

• Fix stash namespace deletion: delete $::{'Foo::'} was a complete no-op because RuntimeStash.deleteGlob() computed fullKey = "main::Foo::" but symbols are stored as "Foo::bar" (without the main:: prefix). The exact-key lookup found nothing and returned early.
• Add deleteNamespace() method in RuntimeStash: When the key ends with ::, performs prefix-based removal from all 6 global maps (globalCodeRefs, globalVariables, globalArrays, globalHashes, globalIORefs, globalFormatRefs)
• Add clearPinnedCodeRefsForNamespace() in GlobalVariable: Prevents deleted subs from being resurrected by getGlobalCodeRef() lookups (which re-inserts from pinnedCodeRefs)
• Invalidate caches after namespace deletion (method resolution + package existence)

Before

$ ./jperl -w -e 'package Foo; sub bar { 42 } package main; delete $::{"Foo::"}; print Foo->can("bar") ? "yes" : "no"'
yes    # WRONG - stash delete was a no-op

After

$ ./jperl -w -e 'package Foo; sub bar { 42 } package main; delete $::{"Foo::"}; print Foo->can("bar") ? "yes" : "no"'
no     # Correct - namespace symbols removed

This fixes spurious Subroutine ACTION_foo redefined warnings in Module::Build's t/help.t test, which calls delete($::{'MyModuleBuilder::'}) between test blocks to clean up the namespace.

Test plan

• Reproducer: Foo->can('bar') returns false after delete $::{'Foo::'}
• No 'Subroutine redefined' warning after stash delete + redefine
• Module::Build t/help.t: 23/23 pass, zero warnings
• Module::Build full suite: 53/53 test files, 1155/1155 subtests pass
• make passes (all unit tests)

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