PDF: const-qualify scalar by-value constructor params

Add top-level const to the cheap scalar by-value parameters `n`
(ExponentialFunction) and `bits_per_sample` (SampledFunction), matching
the codebase convention of const-qualifying scalar params in definitions.
Object/vector params stay non-const because they are std::move'd into
members, so adding const would defeat the move.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Uqx1dUpDUykRxui8FYwxvM

Author: andiwand

src/odr/internal/pdf/pdf_function.cpp

@@ -46,7 +46,8 @@ double interpolate(const double x, const double a0, const double a1,
 class ExponentialFunction final : public Function {
 public:
   ExponentialFunction(std::vector<double> domain, std::vector<double> range,
-                      std::vector<double> c0, std::vector<double> c1, double n)
+                      std::vector<double> c0, std::vector<double> c1,
+                      const double n)
       : Function(std::move(domain), std::move(range)), m_c0{std::move(c0)},
         m_c1{std::move(c1)}, m_n{n} {}
 
@@ -115,7 +116,8 @@ class StitchingFunction final : public Function {
 class SampledFunction final : public Function {
 public:
   SampledFunction(std::vector<double> domain, std::vector<double> range,
-                  std::vector<std::size_t> size, std::int32_t bits_per_sample,
+                  std::vector<std::size_t> size,
+                  const std::int32_t bits_per_sample,
                   std::vector<double> encode, std::vector<double> decode,
                   std::string samples)
       : Function(std::move(domain), std::move(range)), m_size{std::move(size)},