Replace `tool/update-deps` with a Ruby source scanner so dependency
updates no longer require GNU make, a configured build, or compiler
preprocessor output.
Keep compact source mappings and scanner declarations in dependency
files, and expand complete rules into the build directory. Avoid the
temporary fixture headers required by the compiler-driven prototype.
Use the generated rules from GNU make, BSD make, and NMake builds.
Support out-of-tree and read-only source trees, and retain expanded
dependencies shipped in release archives when baseruby is unavailable.
`File.join` is a hotspot for common libraries such as Zeitwerk
and Bootsnap. It has a fairly flexible signature, but 99% of
the time it's called with just two (or a small number of) UTF-8 strings.
If we optimistically optimize for that use case we can cut down a large
number of type and encoding checks, significantly speeding up the method.
The one remaining expensive check we could try to optimize is `str_null_check`.
Given it's common to use the same base string for joining, we could memoize it.
Also we could precompute it for literal strings.
```
compare-ruby: ruby 4.1.0dev (2026-01-17T14:40:03Z master 00a3b71eaf) +PRISM [arm64-darwin25]
built-ruby: ruby 4.1.0dev (2026-01-18T12:10:38Z spedup-file-join 069bab58d4) +PRISM [arm64-darwin25]
warming up....
| |compare-ruby|built-ruby|
|:-------------|-----------:|---------:|
|two_strings | 2.475M| 9.444M|
| | -| 3.82x|
|many_strings | 551.975k| 2.346M|
| | -| 4.25x|
|array | 514.946k| 522.034k|
| | -| 1.01x|
|mixed | 621.236k| 633.189k|
| | -| 1.02x|
```
Nobuyoshi Nakada