fix(derive): separate value metadata from parsing

[jdx]

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Note

Medium Risk
Behavior changes for enums that relied on derived FromStr without an explicit impl (compile failures until fixed); cfg-aware duplicate detection is heuristic and could mis-allow or mis-reject edge cases.

Overview
ValueEnum is metadata-only. The derive no longer emits impl FromStr; types must supply their own parser, like other typed fields. Help, completions, and choice checks still read the static CHOICES / alias tables from the ValueEnum trait impl.

Cfg-gated variants are allowed. Variant cfg / cfg_attr gates are copied onto static table entries instead of being rejected, and duplicate CLI words are permitted when the variants’ cfg predicates are disjoint (e.g. #[cfg(windows)] vs #[cfg(not(windows))]).

Call sites and tests add hand-written FromStr implementations (lint’s OutputFormat, conformance Interpreter, facade Shell, etc.). PLAN.md marks the fleet launch-gate item for ValueEnum + domain parsing as done.

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Instruction counts

benchmark	trend	instructions	Δ	wall (min)	Δ
markdown	▂▂▃▂▂▃▃▃▁█▇█▇	198,800,919 → 198,756,571	-0.02%	18.77 → 18.44ms	-1.76%
startup	▁▁▁▁▁▁▁▁▁████	1,214,930 → 1,214,960	+0.00%	1.25 → 1.27ms	+1.72%

No instruction-count regression above 1%.

_{Only instruction counts gate. Wall clock is shown for context — on identical hardware it moves 4-20% run to run.}

_{Measured by tak — instruction-counted CLI benchmarks, stored in this repository's git notes.}

Shadow comparison

Parsing mise use -g node@20 against a shadow of mise's committed spec.
Reported, not gated: the shadow grows as the derive learns to express more, so
what to watch is the ratio rather than either column.

framework	instructions, cold parse	vs usage
usage	4253	—
argh	6292	1.5x
clap	5893640	1385x
bpaf	21917948	5153x

                                              min       p01       p10    median
usage-rs: argv -> struct                      188       191       194       199  ns
argh: argv -> struct                          272       277       281       288  ns
clap: build tree + parse -> struct         484328    486430    489046    495484  ns
bpaf: build parser + parse -> struct      1591424   1591424   1599070   1609130  ns

usage: argv -> struct                             200 ns      0.20 µs
clap: build tree + parse -> struct             492941 ns    492.94 µs
clap: parse -> struct, tree reused              23085 ns     23.09 µs
clap: build tree only                          307740 ns    307.74 µs

_{704bc20743e9 vs 93ea42d8d54e · measured on the runner, not pushed to the history.}

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Cursor Bugbot has reviewed your changes and found 1 potential issue.

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