docs(cpp14): add 01-relaxed-constexpr book chapter, exercises, solutions, and build wiring

[lczllx]

docs(cpp14): add 01-relaxed-constexpr book chapter, exercises, solutions, and build wiring

Add the second C++14 chapter covering relaxed constexpr.

Book chapter (zh + en):

• section 一/I: Basic usage — for/while loops, if/switch branches,
  local variables, multiple statements, compile-time Fibonacci
• section 二/II: Real-world case — std::make_index_sequence and
  std::tuple compile-time construction, cited from vendored
  msvc-stl/stl/inc/tuple and msvc-stl/stl/inc/utility
• section 三/III: Notes — still-banned operations (goto, try/catch,
  static locals, asm, uninitialized vars), dual nature of constexpr,
  ODR linkage note (implicit inline is C++17, not C++14)
• section 四/IV: Exercise topics and d2x checker command
• section 五/V: External resources

Exercise progression:
0. constexpr with loops — compile-time factorial (for) and power
(while). Learner fills loop condition and return expressions.
4 D2X_YOUR_ANSWER.

1. constexpr with branches and local variables — if-branch my_abs,
   switch-branch days_in_month, multi-variable Fibonacci. Learner
   fills branch conditions, switch default, initial values and
   return expressions. 6 D2X_YOUR_ANSWER.

Design rationale:
The jump from C++11 single-return constexpr to C++14 loop/branch
constexpr is the single most impactful constexpr change. ex 0
isolates loop relaxation — factorial uses for, power uses while,
both forms C++11 forbade. ex 1 layers on branching and multiple
local variables, showing that C++14 constexpr functions can look
like normal functions.

The STL case uses std::make_index_sequence + std::tuple because
index_sequence was introduced in C++14 alongside relaxed constexpr
and is the prime example of compile-time integer generation enabling
pack expansion — a pattern unavailable in C++11.

Closes #61

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