docs: Add Task #67 for DCT/FFT performance benchmarking

Adds low-priority task to measure and compare DCT/IDCT performance:

**Goal:** Quantify performance differences between implementations
- Reference O(N²) naive DCT/IDCT
- Current FFT-based O(N log N) implementation
- Future SIMD-optimized versions (when written)

**Location:** test_dct.cc or test_fft.cc

**Measurements:**
- Average time per transform (microseconds)
- Throughput (transforms per second)
- Speedup factor vs reference
- Multiple test sizes (128, 256, 512, 1024) for scaling analysis

**Implementation:**
- std::chrono::high_resolution_clock for timing
- 1000+ iterations to reduce noise
- Min/avg/max statistics
- Guarded with !STRIP_ALL for zero production impact

**Benefits:**
- Validate FFT speedup claims (O(N log N) vs O(N²))
- Quantify SIMD optimization gains when implemented
- Detect performance regressions in CI

**Priority:** Very Low (informational, not blocking any features)

This will be useful when optimizing audio performance in Phase 2.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

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