fix(run): match environment variable names by platform rules

[wan9chi]

Motivation

Environment variable names ignore ASCII case on Windows, but the env maps used to plan, spawn, and serve tasks compared names exactly:

• For task command assignments such as Foo=a FOO=b, map iteration order decided which value a task saw, and an assignment could fail to replace an inherited variable spelled differently.
• The FORCE_COLOR fallback added a second entry next to a differently spelled parent value.
• VP_RUN_CONCURRENCY_LIMIT and runner getEnv lookups missed differently spelled variables.
• PATH lookups and prefix queries handled case by hand in several places.

Summary

• Add vt_casefold. Folded<S, R> compares a string by the case rule R while keeping its original spelling. EnvName<S> uses the platform's rule (ASCII letters ignore case on Windows; exact elsewhere), and EnvName<OsStr> is the borrowed form for map lookups.
• Key the env maps in vt_shell, vt_plan, vt, vt_server, and fspy::Command by EnvName, replacing the manual case handling.
• When an assignment overrides a variable spelled differently, the task gets the assignment's spelling, and its fingerprint entry replaces the one recorded under the other spelling. Cache keys and fingerprints still store plain strings.
• TaskParsedCommand drops its unused wincode and Diff derives, which don't support EnvName keys.
• Session::init_with, ScriptCommand.envs, SyntheticPlanRequest.envs, SpawnCommand.spawn_envs, and get_path_env now use EnvName keys; callers depend on vt_casefold directly.

Non-ASCII letters still compare exactly, as documented on EnvNameCaseRule.

[github-actions]

fspy benchmark

linux

dynamic/launch             change  -1.38%  [-11.56% .. +12.32%]  overhead  +306.75%
dynamic/access             change  +1.61%  [ -3.93% .. +10.70%]  overhead   +20.44%
dynamic/access-relative    change  +0.29%  [ -5.28% .. +15.90%]  overhead   +65.45%
dynamic/access-contended   change  -7.70%  [-21.13% .. +15.67%]  overhead    +4.08%
static/launch              change  +0.32%  [-13.05% .. +14.11%]  overhead  +697.97%
static/access              change  +0.94%  [ -9.31% ..  +8.22%]  overhead +1402.77%
static/access-relative     change  -0.39%  [-15.31% ..  +7.74%]  overhead +1829.09%
static/access-contended    change  -1.15%  [-11.14% ..  +7.72%]  overhead +1220.23%

macos

dynamic/launch             change  +0.07%  [ -3.08% ..  +2.57%]  overhead  +214.20%
dynamic/access             change  -0.37%  [ -3.63% ..  +3.37%]  overhead    +2.23%
dynamic/access-relative    change  +0.42%  [ -3.68% ..  +3.36%]  overhead  +253.14%
dynamic/access-contended   change  -2.01%  [-15.47% ..  +7.55%]  overhead    +4.66%

windows

dynamic/launch             change  -1.43%  [ -9.35% ..  +9.38%]  overhead   +23.28%
dynamic/access             change  +0.18%  [ -1.82% ..  +2.86%]  overhead    +1.70%
dynamic/access-relative    change  +0.18%  [ -1.09% ..  +1.62%]  overhead    +1.83%
dynamic/access-contended   change  +0.11%  [ -9.08% ..  +4.10%]  overhead    +3.26%