docs(cpp14): add 00-generic-lambdas book chapter, exercises, solutions, and build wiring

[lczllx]

docs(cpp14): add 00-generic-lambdas book chapter, exercises, solutions, and build wiring

Add the first C++14 chapter covering generic lambdas.

Book chapter (zh + en):

• section 一/I: Basic usage — identity, multi-param, STL algorithms,
  captures, returning lambda (factory pattern)
• section 二/II: Notes — closure type uniqueness, perfect forwarding,
  non-variadic
• section 三/III: Exercise topics and d2x checker command
• section 四/IV: External resources
• register chapter in zh/en SUMMARY.md and book/README.md

Exercise progression:
0. Basic generic lambda — auto parameter identity/comparison/type-size
exercises. Learner fills auto param type, return expression, and
sizeof argument. 4 D2X_YOUR_ANSWER.

1. Generic lambda with STL algorithms — same lambda reused across
   vector and vector for sort/find_if, plus a factory
   pattern returning a lambda. 5 D2X_YOUR_ANSWER.

Design rationale:
Generic lambdas have three layers worth teaching independently:
(1) syntax — auto in lambda parameters generates a templated operator();
(2) reuse — a single lambda serves multiple container types, the
primary motivation for the feature;
(3) interaction — generic lambda returns + captures work naturally
without extra syntax.

ex 0 isolates layer (1). identity forces the learner to write auto
in a parameter position. greater verifies the return expression is
type-checked per instantiation. get_type_size confirms that sizeof
on a generic parameter works correctly, reinforcing the "compiler
stamps out per-type copies" mental model.

ex 1 layers (2) and (3) on top. The desc lambda applied to both
vector and vector makes the payoff tangible. The find_if
exercise adds a capture without complicating the generic parameter.
The make_multiplier factory previews generic-lambda-returning-lambda,
which is a pattern that only becomes practical with C++14 return type
deduction.

We deliberately limited this chapter to 2 exercises. A third exercise
covering perfect forwarding + auto&& + decltype(auto) is better placed
in the decltype(auto) chapter where the forwarding semantics are the
primary topic, not a side note.

Build wiring:

• register cpp14 in dslings/xmake.lua, dslings/en/xmake.lua,
  solutions/xmake.lua
• new xmake.lua per directory with set_languages("cxx14")
• remove stale dslings/cpp14/README.md TODO placeholder

Closes #60

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[Copilot]

Pull request overview

Adds the first C++14 chapter (generic lambdas) with accompanying exercises, reference solutions, and xmake build wiring so the new material can be built/checked alongside existing chapters.

Changes:

• Added new C++14 “00 - generic lambdas” chapter in both zh/en books and registered it in both SUMMARY.md files.
• Introduced two new exercise files (zh + en) plus corresponding reference solutions.
• Wired cpp14 targets into existing xmake build structure for dslings and solutions; removed the stale cpp14 README placeholder.

Reviewed changes

Copilot reviewed 18 out of 18 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
solutions/xmake.lua	Includes cpp14 solutions build script.
solutions/cpp14/xmake.lua	Defines xmake targets for cpp14 reference solution binaries.
solutions/cpp14/00-generic-lambdas-0.cpp	Reference solution for exercise 0.
solutions/cpp14/00-generic-lambdas-1.cpp	Reference solution for exercise 1.
dslings/xmake.lua	Includes cpp14 exercises in zh build.
dslings/en/xmake.lua	Includes cpp14 exercises in en build.
dslings/cpp14/xmake.lua	Defines zh cpp14 exercise targets.
dslings/en/cpp14/xmake.lua	Defines en cpp14 exercise targets.
dslings/cpp14/README.md	Removes the old TODO placeholder file.
dslings/cpp14/00-generic-lambdas-0.cpp	New zh exercise 0 scaffold with D2X placeholders.
dslings/cpp14/00-generic-lambdas-1.cpp	New zh exercise 1 scaffold with D2X placeholders.
dslings/en/cpp14/00-generic-lambdas-0.cpp	New en exercise 0 scaffold with D2X placeholders.
dslings/en/cpp14/00-generic-lambdas-1.cpp	New en exercise 1 scaffold with D2X placeholders.
book/src/SUMMARY.md	Registers the new zh cpp14 chapter in the book TOC.
book/src/cpp14/00-generic-lambdas.md	Adds the zh generic lambdas chapter content.
book/en/src/SUMMARY.md	Registers the new en cpp14 chapter in the book TOC.
book/en/src/cpp14/00-generic-lambdas.md	Adds the en generic lambdas chapter content.
book/README.md	Adds cpp14 cppreference links to the book README.

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