[Backport #21941]

In case of `--yjit-disable`, YJIT only starts to record environment
escapes after `RubyVM::YJIT.enable`. Previously we falsely assumed that
we always have a full history all the way back to VM boot. This had YJIT
install and run code that assume EP=BP when EP≠BP for some exceptional
entry into the middle of a running frame, if the environment escaped
before `YJIT.enable`.

The fix is to reject exceptional entry with an escaped environment.
Rename things and explain in more detail how the predicate for deciding
to assume EP=BP works. It's quite subtle since it reasons about all
parties in the system that push a control frame and then run JIT code.

Note that while can_assume_on_stack_env() checks the currently running
environment if it so happens to be the one YJIT is compiling against, it
can return true for any ISEQ. The check isn't necessary for fixing the
bug, and the load bearing part of this patch is the change to
exceptional entries.

This fix is flat on speed and space on ruby-bench headline benchmarks.

Many thanks for the community effort to create a small test case for
this bug.

Re-organize page docs

When these instructions were introduced it was common to read from a
hash with mutable string literals.  However, these days, I think these
instructions are fairly rare.

I tested this with the lobsters benchmark, and saw no difference in
speed.  In order to be sure, I tracked down every use of this
instruction in the lobsters benchmark, and there were only 4 places
where it was used.

Additionally, this patch fixes a case where "chilled strings" should
emit a warning but they don't.

```ruby
class Foo
  def self.[](x)= x.gsub!(/hello/, "hi")
end

Foo["hello world"]
```

Removing these instructions shows this warning:

```
> ./miniruby -vw test.rb
ruby 3.5.0dev (2025-08-25T21:36:50Z rm-opt_aref_with dca08e286c) +PRISM [arm64-darwin24]
test.rb:2: warning: literal string will be frozen in the future (run with --debug-frozen-string-literal for more information)
```

[Feature #21553]

`ruby --yjit --zjit` already warns and exits, but it was still possible
to enable both with `ruby --zjit -e 'RubyVM:YJIT.enable`.

This commit prevents that with a warning and an early return. (We could
also exit, but that seems a bit unfriendly once we're already running
the program.)

Co-authored-by: ywenc <ywenc@github.com>

* first commit

* yjit.rb change

* revert formatting

* rename mem-size to exec-mem-size for correctness

* wip, move setting into rb_yjit_enable directly

* remove unused helper functions

* add in call threshold

* input validation with extensive eprintln

* delete test script

* exec-mem-size -> mem-size

* handle input validation with asserts

* add test cases related to input validation

* modify test cases

* move validation out of rs, into rb

* add comments

* remove trailing spaces

* remove logging

Co-authored-by: Takashi Kokubun <takashikkbn@gmail.com>

* remove helper fn

* Update test/ruby/test_yjit.rb

Co-authored-by: Takashi Kokubun <takashikkbn@gmail.com>

* trailing white space

---------

Co-authored-by: Alan Wu <XrXr@users.noreply.github.com>
Co-authored-by: Takashi Kokubun <takashikkbn@gmail.com>
Co-authored-by: Maxime Chevalier-Boisvert <maxime.chevalierboisvert@shopify.com>