docs: align XLA preflight docs with xla 0.10 cuda13 support

Critical review found three stale references that still described
XLA_TARGET=cuda13 as rejected by xla 0.9.x after the v0.2.0 dep-stack
bump. The code and main troubleshooting guide were already updated;
this commit fixes the remaining packaged docs/comments:

- Mix.Tasks.Compile.XlaEnvPreflight moduledoc now uses cuda14 as the
  unsupported example and names xla 0.10.0 / cuda13 in the accepted set.
- XlaTargetValidator error message points at the renamed troubleshooting
  heading.
- docs/production_runbook.md now says cuda13 is accepted under xla
  0.10.x and cuda12 remains recommended.

Verification:
- mix test test/source_policy_test.exs test/build_support/xla_target_validator_test.exs
- mix docs --warnings-as-errors

Author: nshkrdotcom

build_support/mix_tasks_compile_xla_env_preflight.exs

@@ -8,12 +8,12 @@ defmodule Mix.Tasks.Compile.XlaEnvPreflight do
   ## Why this exists
 
   Without this preflight, an unrecognised `XLA_TARGET` (for example
-  `cuda13` against the bundled `xla 0.9.x`) surfaces as a `RuntimeError`
+  `cuda14` against the bundled `xla 0.10.x`) surfaces as a `RuntimeError`
   stacktrace from `deps/exla/mix.exs` during dependency compilation:
 
       ** (RuntimeError) expected XLA_TARGET to be one of
-         "cpu", "cuda", "rocm", "tpu", "cuda12", but got: "cuda13"
-          (xla 0.9.1) lib/xla.ex:82: XLA.xla_target/0
+         "cpu", "cuda", "rocm", "tpu", "cuda12", "cuda13", but got: "cuda14"
+          (xla 0.10.0) lib/xla.ex:82: XLA.xla_target/0
           ...
 
   With this preflight, the project surfaces the failure with a single

build_support/xla_target_validator.exs

@@ -83,8 +83,8 @@ defmodule XlaTargetValidator do
         "Recommended for CPU hosts: unset XLA_TARGET (or use cpu). " <>
         "The bundled xla rejects unrecognised targets at compile time, so EXLA " <>
         "cannot compile until XLA_TARGET is corrected. See " <>
-        "guides/troubleshooting.md (\"XLA_TARGET=cuda13 Is Rejected At Compile " <>
-        "Time\") for the canonical recipe."
+        "guides/troubleshooting.md (\"XLA_TARGET Rejected At Compile Time\") " <>
+        "for the canonical recipe."
     )
   end
 end

docs/production_runbook.md

@@ -29,8 +29,10 @@ Common output:
 trinity.env.check: ok / xla_target=cuda12
 ```
 
-Known gotcha: `XLA_TARGET=cuda13` is rejected at compile time by
-`xla 0.9.1`. Use `cuda12`.
+Known gotcha: `XLA_TARGET` is validated before EXLA compiles. With the
+current `xla 0.10.x` stack, `cuda13` is accepted; `cuda12` remains the
+recommended default for CUDA hosts. Unsupported values such as `cuda14` fail
+fast with a readable `Mix.Error`.
 
 ## 2. Canonical artifact