Add String#view core extension

[paracycle]

Summary

Adds an opt-in String#view method via require "string_view/core_ext" that creates pool-backed StringView objects directly from any string:

require "string_view/core_ext"

buf = "Hello, World!"
buf.view(0, 5)   # => StringView "Hello"
buf.view(7, 5)   # => StringView "world"

Implementation

• Defines StringView::CoreExt module in C (string_view_core_ext.c) with a view(byte_offset, byte_length) method
• lib/string_view/core_ext.rb includes the module into String
• Pools are cached per-string in an ObjectSpace::WeakKeyMap — automatically cleaned up when the string is GC'd
• Calls pool_view directly in C (no Ruby method dispatch on the hot path); rb_funcall only on cache miss (once per string)
• String#view implicitly freezes the string before creating the pool (required for safe views)

Changes

• New: ext/string_view/string_view_core_ext.c, lib/string_view/core_ext.rb, test/test_core_ext.rb
• string_view_pool.c: cStringViewPool and pool_view made non-static for cross-file access; pool refreshes cached base pointer on each view call
• string_view.h: added extern cStringViewPool, pool_view, and Init_string_view_core_ext declarations
• string_view.c: calls Init_string_view_core_ext() from Init_string_view
• extconf.rb: added string_view_core_ext.c to sources
• README.md: documented the String#view extension
• test_string_method_coverage.rb: excludes CoreExt methods from coverage check