feat(cpp14): add 02-variable-templates book chapter, exercises, solutions, and build wiring - #76

feat(cpp14): add 02-variable-templates book chapter, exercises, solutions, and build wiring

Add the third C++14 chapter covering variable templates.

Book chapter (zh + en):

• section 一/I: Basic usage — pi constant, _v traits, compile-time
  config values, recursive variable template
• section 二/II: Real-world case — _Is_iterator_v from vendored
  msvc-stl/stl/inc/xutility, demonstrating variable template with
  partial specialization replacing ::value pattern
• section 三/III: Notes — full and partial specialization support
• section 四/IV: Exercise topics and d2x checker command
• section 五/V: External resources

Exercise progression:
0. Basic variable template — pi constant and compile-time buffer
config with explicit specialization. 4 D2X_YOUR_ANSWER.

1. _v type traits — implementing is_void_v, is_same_v, and
   recursive factorial_v. 5 D2X_YOUR_ANSWER.

STL case notes:
The vendored _Is_iterator_v was chosen because it (1) is a real
variable template used internally by the MSVC STL, (2) demonstrates
partial specialization of a variable template, and (3) directly
illustrates the motivation — eliminating ::value boilerplate from
type traits. This predates and motivates the C++17 standardized _v
traits like is_same_v.

Build wiring:

• register 02 targets in dslings/cpp14, dslings/en/cpp14,
  solutions/cpp14
• add 02 entry to zh/en SUMMARY.md

Closes #62

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