fix(teensy41): link libarm_cortexM7lfsp_math (CMSIS-DSP)

Closes #257.

`Ports/PJRCSpectrumAnalyzer` and any other Teensy Audio FFT example
fails to link on teensy41 with:

    undefined reference to `arm_cfft_radix4_init_q15'
    undefined reference to `arm_cfft_radix4_q15'

These symbols live in `libarm_cortexM7lfsp_math.a`, shipped by the
Teensy 4.x core. Teensyduino's `boards.txt` specifies
`build.flags.libs=-larm_cortexM7lfsp_math -lm -lstdc++` mandatorily
for the platform — it's not example-specific.

fbuild's teensy4x.json `linker_libs` was missing the CMSIS-DSP entry.
Add `-larm_cortexM7lfsp_math` to the front of the list so it precedes
the standard libs (libgcc / libstdc++ / libm / libc). The `-L<core_dir>`
search path is already emitted via `LinkerScripts::single(core_dir, ...)`
in teensy/orchestrator.rs:244 (the same dir that holds the .ld script
also holds the lib), so no additional path config is required.

Regression test `teensy4x_links_cmsis_dsp_math` added to
`teensy/mcu_config.rs`.

Note: the PlatformIO reference JSON for teensy41
(crates/fbuild-build/src/teensy/configs/reference/teensy41.json) also
omits this lib — PlatformIO appears to have the same gap. Following
Teensyduino's boards.txt rather than PlatformIO's reference is the
correct call per the issue analysis.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: zackees

crates/fbuild-build/src/teensy/configs/teensy4x.json

@@ -38,6 +38,7 @@
   ],
 
   "linker_libs": [
+    "-larm_cortexM7lfsp_math",
     "-lgcc",
     "-lstdc++",
     "-lm",

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

@@ -199,6 +199,26 @@ mod tests {
             .contains(&"-mcpu=cortex-m7".to_string()));
     }
 
+    /// Regression test for issue #257: teensy4x must link
+    /// `libarm_cortexM7lfsp_math.a` (CMSIS-DSP) so Teensy Audio FFT
+    /// examples (`Ports/PJRCSpectrumAnalyzer`) resolve symbols like
+    /// `arm_cfft_radix4_q15`. The library ships in the Teensy 4.x core
+    /// dir, which the linker already gets as `-L<core_dir>` via
+    /// `LinkerScripts::single`.
+    #[test]
+    fn teensy4x_links_cmsis_dsp_math() {
+        let config = get_teensy_config_for_mcu("imxrt1062").expect("teensy4x config");
+        assert!(
+            config
+                .linker_libs
+                .contains(&"-larm_cortexM7lfsp_math".to_string()),
+            "teensy4x linker_libs must include -larm_cortexM7lfsp_math \
+             so Teensy Audio FFT examples link; see issue #257. \
+             Actual libs: {:?}",
+            config.linker_libs
+        );
+    }
+
     #[test]
     fn test_teensy_config_for_mcu_mk20dx256() {
         let config = get_teensy_config_for_mcu("mk20dx256").expect("teensy3x config");