fix(teensy): derive map_path from elf_path to fix output_dir scope error (#316)

`build_link_args` was added by #313 (CMSIS-DSP auto-link refactor) but a
hold-over from #305 still referenced `output_dir.join("firmware.map")`,
which doesn't exist as a parameter of `build_link_args` (it takes
`elf_path: &Path`, not `output_dir`). The workspace failed to compile
with:

    error[E0425]: cannot find value `output_dir` in this scope
      --> crates/fbuild-build/src/teensy/teensy_linker.rs:93:24

Every PR opened against main since #313 has been red because of this.

Derive the map path from `elf_path.with_extension("map")`, which is
equivalent to `output_dir.join("firmware.map")` since
`elf_path = output_dir.join("firmware.elf")` at the only call site.

Adds `test_teensy_link_command_emits_linker_map_next_to_elf` so the
regression is caught locally next time.

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

Author: zackees

crates/fbuild-build/src/teensy/teensy_linker.rs

@@ -90,7 +90,7 @@ impl TeensyLinker {
         args.extend(["-o".to_string(), elf_path.to_string_lossy().to_string()]);
 
         // Always emit a linker map next to firmware.elf for debugging (#305).
-        let map_path = output_dir.join("firmware.map");
+        let map_path = elf_path.with_extension("map");
         args.push(format!("-Wl,-Map={}", map_path.to_string_lossy()));
 
         // Sketch objects first
@@ -325,6 +325,37 @@ mod tests {
         );
     }
 
+    /// Regression test: `build_link_args` always emits `-Wl,-Map=` next to
+    /// the elf path so debug builds can inspect link decisions. Reverted
+    /// previously by an out-of-scope `output_dir` reference (see #313 hotfix).
+    #[test]
+    fn test_teensy_link_command_emits_linker_map_next_to_elf() {
+        let linker = TeensyLinker::new(
+            PathBuf::from("/bin/arm-none-eabi-gcc"),
+            PathBuf::from("/bin/arm-none-eabi-ar"),
+            PathBuf::from("/bin/arm-none-eabi-objcopy"),
+            PathBuf::from("/bin/arm-none-eabi-size"),
+            LinkerScripts::single(PathBuf::from("/teensy4"), "imxrt1062_t41.ld"),
+            crate::teensy::mcu_config::get_teensy_config_for_mcu("imxrt1062").unwrap(),
+            BuildProfile::Release,
+            Some(8126464),
+            Some(524288),
+            None,
+            false,
+        );
+        let args = linker.build_link_args(
+            &[],
+            &[],
+            &PathBuf::from("/build/firmware.elf"),
+            &LinkExtraArgs::default(),
+        );
+        assert!(
+            args.iter().any(|a| a == "-Wl,-Map=/build/firmware.map"),
+            "expected -Wl,-Map=/build/firmware.map next to firmware.elf. Args: {:?}",
+            args
+        );
+    }
+
     /// Boards that do not declare a CMSIS-DSP lib (e.g. user override clears
     /// it) must not have a spurious `-l` argument appended.
     #[test]