Optimize File.join common use case

`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|
```

1 files changed, 15 insertions, 0 deletions

diff --git a/ext/socket/depend b/ext/socket/depend
index 3573dc45e2..77f6239a3d 100644
--- a/ext/socket/depend
+++ b/ext/socket/depend
@@ -193,6 +193,7 @@ ancdata.o: $(top_srcdir)/ccan/check_type/check_type.h
 ancdata.o: $(top_srcdir)/ccan/container_of/container_of.h
 ancdata.o: $(top_srcdir)/ccan/list/list.h
 ancdata.o: $(top_srcdir)/ccan/str/str.h
+ancdata.o: $(top_srcdir)/encindex.h
 ancdata.o: $(top_srcdir)/id_table.h
 ancdata.o: $(top_srcdir)/internal.h
 ancdata.o: $(top_srcdir)/internal/array.h
@@ -408,6 +409,7 @@ basicsocket.o: $(top_srcdir)/ccan/check_type/check_type.h
 basicsocket.o: $(top_srcdir)/ccan/container_of/container_of.h
 basicsocket.o: $(top_srcdir)/ccan/list/list.h
 basicsocket.o: $(top_srcdir)/ccan/str/str.h
+basicsocket.o: $(top_srcdir)/encindex.h
 basicsocket.o: $(top_srcdir)/id_table.h
 basicsocket.o: $(top_srcdir)/internal.h
 basicsocket.o: $(top_srcdir)/internal/array.h
@@ -623,6 +625,7 @@ constants.o: $(top_srcdir)/ccan/check_type/check_type.h
 constants.o: $(top_srcdir)/ccan/container_of/container_of.h
 constants.o: $(top_srcdir)/ccan/list/list.h
 constants.o: $(top_srcdir)/ccan/str/str.h
+constants.o: $(top_srcdir)/encindex.h
 constants.o: $(top_srcdir)/id_table.h
 constants.o: $(top_srcdir)/internal.h
 constants.o: $(top_srcdir)/internal/array.h
@@ -839,6 +842,7 @@ ifaddr.o: $(top_srcdir)/ccan/check_type/check_type.h
 ifaddr.o: $(top_srcdir)/ccan/container_of/container_of.h
 ifaddr.o: $(top_srcdir)/ccan/list/list.h
 ifaddr.o: $(top_srcdir)/ccan/str/str.h
+ifaddr.o: $(top_srcdir)/encindex.h
 ifaddr.o: $(top_srcdir)/id_table.h
 ifaddr.o: $(top_srcdir)/internal.h
 ifaddr.o: $(top_srcdir)/internal/array.h
@@ -1054,6 +1058,7 @@ init.o: $(top_srcdir)/ccan/check_type/check_type.h
 init.o: $(top_srcdir)/ccan/container_of/container_of.h
 init.o: $(top_srcdir)/ccan/list/list.h
 init.o: $(top_srcdir)/ccan/str/str.h
+init.o: $(top_srcdir)/encindex.h
 init.o: $(top_srcdir)/id_table.h
 init.o: $(top_srcdir)/internal.h
 init.o: $(top_srcdir)/internal/array.h
@@ -1269,6 +1274,7 @@ ipsocket.o: $(top_srcdir)/ccan/check_type/check_type.h
 ipsocket.o: $(top_srcdir)/ccan/container_of/container_of.h
 ipsocket.o: $(top_srcdir)/ccan/list/list.h
 ipsocket.o: $(top_srcdir)/ccan/str/str.h
+ipsocket.o: $(top_srcdir)/encindex.h
 ipsocket.o: $(top_srcdir)/id_table.h
 ipsocket.o: $(top_srcdir)/internal.h
 ipsocket.o: $(top_srcdir)/internal/array.h
@@ -1484,6 +1490,7 @@ option.o: $(top_srcdir)/ccan/check_type/check_type.h
 option.o: $(top_srcdir)/ccan/container_of/container_of.h
 option.o: $(top_srcdir)/ccan/list/list.h
 option.o: $(top_srcdir)/ccan/str/str.h
+option.o: $(top_srcdir)/encindex.h
 option.o: $(top_srcdir)/id_table.h
 option.o: $(top_srcdir)/internal.h
 option.o: $(top_srcdir)/internal/array.h
@@ -1699,6 +1706,7 @@ raddrinfo.o: $(top_srcdir)/ccan/check_type/check_type.h
 raddrinfo.o: $(top_srcdir)/ccan/container_of/container_of.h
 raddrinfo.o: $(top_srcdir)/ccan/list/list.h
 raddrinfo.o: $(top_srcdir)/ccan/str/str.h
+raddrinfo.o: $(top_srcdir)/encindex.h
 raddrinfo.o: $(top_srcdir)/id_table.h
 raddrinfo.o: $(top_srcdir)/internal.h
 raddrinfo.o: $(top_srcdir)/internal/array.h
@@ -1914,6 +1922,7 @@ socket.o: $(top_srcdir)/ccan/check_type/check_type.h
 socket.o: $(top_srcdir)/ccan/container_of/container_of.h
 socket.o: $(top_srcdir)/ccan/list/list.h
 socket.o: $(top_srcdir)/ccan/str/str.h
+socket.o: $(top_srcdir)/encindex.h
 socket.o: $(top_srcdir)/id_table.h
 socket.o: $(top_srcdir)/internal.h
 socket.o: $(top_srcdir)/internal/array.h
@@ -2129,6 +2138,7 @@ sockssocket.o: $(top_srcdir)/ccan/check_type/check_type.h
 sockssocket.o: $(top_srcdir)/ccan/container_of/container_of.h
 sockssocket.o: $(top_srcdir)/ccan/list/list.h
 sockssocket.o: $(top_srcdir)/ccan/str/str.h
+sockssocket.o: $(top_srcdir)/encindex.h
 sockssocket.o: $(top_srcdir)/id_table.h
 sockssocket.o: $(top_srcdir)/internal.h
 sockssocket.o: $(top_srcdir)/internal/array.h
@@ -2344,6 +2354,7 @@ tcpserver.o: $(top_srcdir)/ccan/check_type/check_type.h
 tcpserver.o: $(top_srcdir)/ccan/container_of/container_of.h
 tcpserver.o: $(top_srcdir)/ccan/list/list.h
 tcpserver.o: $(top_srcdir)/ccan/str/str.h
+tcpserver.o: $(top_srcdir)/encindex.h
 tcpserver.o: $(top_srcdir)/id_table.h
 tcpserver.o: $(top_srcdir)/internal.h
 tcpserver.o: $(top_srcdir)/internal/array.h
@@ -2559,6 +2570,7 @@ tcpsocket.o: $(top_srcdir)/ccan/check_type/check_type.h
 tcpsocket.o: $(top_srcdir)/ccan/container_of/container_of.h
 tcpsocket.o: $(top_srcdir)/ccan/list/list.h
 tcpsocket.o: $(top_srcdir)/ccan/str/str.h
+tcpsocket.o: $(top_srcdir)/encindex.h
 tcpsocket.o: $(top_srcdir)/id_table.h
 tcpsocket.o: $(top_srcdir)/internal.h
 tcpsocket.o: $(top_srcdir)/internal/array.h
@@ -2774,6 +2786,7 @@ udpsocket.o: $(top_srcdir)/ccan/check_type/check_type.h
 udpsocket.o: $(top_srcdir)/ccan/container_of/container_of.h
 udpsocket.o: $(top_srcdir)/ccan/list/list.h
 udpsocket.o: $(top_srcdir)/ccan/str/str.h
+udpsocket.o: $(top_srcdir)/encindex.h
 udpsocket.o: $(top_srcdir)/id_table.h
 udpsocket.o: $(top_srcdir)/internal.h
 udpsocket.o: $(top_srcdir)/internal/array.h
@@ -2989,6 +3002,7 @@ unixserver.o: $(top_srcdir)/ccan/check_type/check_type.h
 unixserver.o: $(top_srcdir)/ccan/container_of/container_of.h
 unixserver.o: $(top_srcdir)/ccan/list/list.h
 unixserver.o: $(top_srcdir)/ccan/str/str.h
+unixserver.o: $(top_srcdir)/encindex.h
 unixserver.o: $(top_srcdir)/id_table.h
 unixserver.o: $(top_srcdir)/internal.h
 unixserver.o: $(top_srcdir)/internal/array.h
@@ -3204,6 +3218,7 @@ unixsocket.o: $(top_srcdir)/ccan/check_type/check_type.h
 unixsocket.o: $(top_srcdir)/ccan/container_of/container_of.h
 unixsocket.o: $(top_srcdir)/ccan/list/list.h
 unixsocket.o: $(top_srcdir)/ccan/str/str.h
+unixsocket.o: $(top_srcdir)/encindex.h
 unixsocket.o: $(top_srcdir)/id_table.h
 unixsocket.o: $(top_srcdir)/internal.h
 unixsocket.o: $(top_srcdir)/internal/array.h