Primitive receivers: method_missing, Symbol#method after undef, block-form reopen #393

Found while fixing the #387 review. Two gaps around primitive receivers, both on master as well (verified 2026-09-24, CRuby 3.4.8 as oracle).

1. A user method_missing is never reached for a Symbol receiver.

class Object; def method_missing(n, *a) = [:mm, n, *a]; end
p :abc.nope(1)   # CRuby: [:mm, :nope, 1]   rubyrs: NoMethodError

The same applies after class Symbol; undef_method :start_with?; end (CRuby: [:mm, :start_with?, "a"]).

2. Symbol#method ignores an undef.

class Symbol; undef_method :length; end
:abc.method(:length)   # CRuby: NameError   rubyrs: #<Method: Symbol#length()>

The call itself raises NoMethodError correctly (the do_call gate from #387); only method() / Method-object creation misses the sym_undefed tombstone check.

3. Block-form calls on primitives ignore a reopen.

class String; def each_char(*) = :own; end
p "ab".each_char { }   # CRuby: :own   rubyrs: "ab"
class Integer; def times = :own; end
p 3.times { }          # CRuby: :own   rubyrs: 3

The block-form reopen gate (the prim_reopen_mask twin in dispatch.rs) catches each_byte, but each_char and Integer#times take an earlier block fast path that runs before it. Prepending a module shows the same symptom.

Acceptance

• A diff fixture covering all three cases; delete it from quarantine once green.