child_process: build the default env block in one native pass - #65325

spawn(), spawnSync() and everything built on them get ~20–24 % faster when no options.env is passed (the common
case), by not going through the process.env proxy once per variable to build the child's environment block.

child_process/child-process-params.js params=1 methodName='spawn'          ***    19.57 %  ±1.73%
child_process/child-process-params.js params=1 methodName='spawnSync'      ***    24.37 %  ±0.74%
child_process/child-process-params.js params=1 methodName='execFile'       ***    20.07 %  ±1.86%
child_process/child-process-params.js params=1 methodName='execFileSync'   ***    23.99 %  ±0.67%
child_process/child-process-params.js params=1 methodName='exec'           ***    17.41 %  ±1.74%
child_process/child-process-params.js params=1 methodName='execSync'       ***    24.03 %  ±0.61%
child_process/spawn-echo.js                                                ***    21.41 %  ±0.61%

(Linux x64, 183-variable environment - the effect scales with environment size; 30 runs.)

normalizeSpawnArguments() did { ...process.env } and walked the copy to build the KEY=value array for uv_spawn().
Spreading the proxy hits a query and a getter interceptor per variable, each a linear scan of the environment under the env
mutex plus an allocation - O(n²), and the largest JS-side cost of a spawn (~190 µs of 1.7 ms here).

This adds KVStore::Pairs() (RealEnvStore: one uv_os_environ() pass, skipping hidden =X: entries on Windows like
Enumerate() does; default: Enumerate() + Get()), exposes it as internalBinding('process_wrap').getEnvPairs(),
and uses it when no options.env is given and the permission model is off. Everything else keeps the existing path.

Notes:

• Same variables and values, same mutex, no caching (a later spawn sees process.env changes). Windows keeps its
  case-insensitive de-duplication.
• POSIX order is now environment-block order; before, integer-like names (a variable literally named 123) were hoisted to
  the front by object key ordering. That is the only observable difference for a normal environment.
• Enumerable properties added to Object.prototype no longer leak into the default child environment. options.env
  objects keep their documented prototype-inclusive behavior.

Tests: test-child-process-default-env.js (new): the child sees exactly the parent's current environment (contents
and, on POSIX, order) via spawnSync, execFileSync and spawn, including variables added/deleted/emptied at runtime,
values with = and non-ASCII; a user env is passed through unmerged; a variable set after one spawn is seen by the next.

Disclosure: the code, test, measurements and this description were written by Claude Code, directed and reviewed by @codebytere.