fix(terraphim_tinyclaw): add SAFETY comment to unsafe set_var in test Refs #2492

`crates/terraphim_tinyclaw/src/config.rs:619` used an `unsafe` block around
`std::env::set_var("TEST_VAR", ...)` without a `// SAFETY:` comment documenting
the invariant. `set_var` is unsafe because concurrent environment reads from
other threads can race with the write.

Add the required SAFETY comment with an accurate invariant: no other test in
this binary reads or writes `TEST_VAR` (verified by grep — the symbol appears
only in this test). This matches the pattern established for
`terraphim_symphony` (#2497, PR #2865) and corrects the imprecise
"single-threaded" claim of the prior stranded commit, since cargo runs tests in
parallel threads by default.

Comment-only change (+3 lines, zero executable code changed).

Verification (all exit 0):
- cargo fmt -p terraphim_tinyclaw -- --check
- cargo clippy -p terraphim_tinyclaw --lib --tests -- -D warnings
- cargo test -p terraphim_tinyclaw --lib  (174 passed, 0 failed)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: Test User

crates/terraphim_tinyclaw/src/config.rs

@@ -616,6 +616,9 @@ model = "llama3.2"
 
     #[test]
     fn test_env_var_expansion() {
+        // SAFETY: No other test in this binary reads or writes TEST_VAR.
+        // Cargo runs tests in parallel threads by default; we accept this because
+        // only this test touches this variable.
         unsafe {
             std::env::set_var("TEST_VAR", "test_value");
         }