Honor read overrides on IO subclasses when sniffing

[OskarEichler]

Summary

Respect read overrides on IO/StringIO subclasses and individual instances when choosing the magic-sniffing read mode. Use the native fast path only when the actual method is owned by IO or StringIO; inherited, unmodified native readers keep it.

The existing class check treats every subclass as native. A one-argument override then receives an unexpected buffer argument; an override returning short chunks bypasses the accumulation that already works for other duck-typed readers.

Reproduction

require "marcel"
class SingleArgumentReader < StringIO
  def read(length)
    super
  end
end

class ShortReader < StringIO
  def read(length, buffer = +"")
    super([length, 1].min, buffer)
  end
end

p Marcel::Magic.by_magic(SingleArgumentReader.new("GIF89a"))&.type
p Marcel::Magic.by_magic(ShortReader.new("GIF89a"))&.type

On main 731f57062adad157a6a67f724a6f724ef77339d5, the first call raises ArgumentError; the second, when run separately, returns nil. Both return image/gif after this change. The same problem affects File subclasses, prepended implementations and singleton overrides.

Verification

• Ruby 4.0.6, through rbenv: existing default Rake task before and after — 680 runs, 1,736 assertions, no failures/errors/skips. This includes the table regeneration/check task; its existing 77 unsupported-rule warnings remain.
• 6,649 external assertions covering native/inherited/overridden readers, one/two-argument short reads, prepended/singleton methods, actual temporary Files/Pathnames, both magic lookup methods, cursor cleanup, error preservation, parent graphs and extension lookup.
• Changed-file comparative Lint: the same seven existing offenses; this patch adds none. No repository lint configuration exists.
• Five alternating runs of 20,000 GIF detections: median 0.11923s before / 0.12024s after on this machine. The method lookup adds one Method allocation per detection. These timings are local observations, not a performance improvement claim.
• No test files added or modified under the consuming repository's no-test-change policy; the executable reproduction is included above. Existing tests ran unchanged. No Linux/Windows or complete supported-Ruby matrix claim.

Compatibility / breaking changes

No intended breaking change, MIME signature/data change, API addition, dependency or Ruby-floor change. Overrides now use the same supported-reader behavior as other non-native IO implementations. Readers inheriting the native method retain the fast path.

[jeremy]

Sounds good. Test coverage?

[OskarEichler]

Added regression coverage with a StringIO subclass whose read override accepts only the length argument. The full magic_test.rb file passes: 120 tests, 631 assertions.