feat(parse): support custom help and version actions

[jdx]

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Note

Medium Risk
Touches core parse, help rendering, and completion walking in both Rust and Go, so regressions would change how help/version is requested. Behavior is additive and gated by explicit flags/actions.

Overview
CLIs can now disable the synthesized --help/-h, help subcommand, and --version/-V entries, and relocate those behaviors onto declared flags via Help, HelpShort, HelpLong, and Version actions.

The same properties travel through typed Rust, KDL, usage-lib, generated Go, and the clap bridge. Help pages omit the synthetic rows when disabled; custom flags keep their own help text. Completion walking no longer treats a declared help/version action as a help-topic parse, so later words still complete.

Disabling these built-ins is no longer a clap-bridge fidelity loss. Derive fields with a non-binding action must be bool flags.

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Instruction counts

benchmark	trend	instructions	Δ	wall (min)	Δ
markdown	█▁▁▃	199,315,003 → 201,208,261	+0.95%	19.00 → 19.18ms	+0.95%
startup	▁▅▅█	1,221,718 → 1,225,360	+0.30%	1.35 → 1.33ms	-1.06%

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	4353	—
argh	6292	1.4x
clap	5893640	1353x
bpaf	21917948	5035x

                                              min       p01       p10    median
usage-rs: argv -> struct                      185       192       193       196  ns
argh: argv -> struct                          299       304       320       338  ns
clap: build tree + parse -> struct         482525    483947    486870    493922  ns
bpaf: build parser + parse -> struct      1623497   1623497   1628202   1637880  ns

usage: argv -> struct                             207 ns      0.21 µs
clap: build tree + parse -> struct             497163 ns    497.16 µs
clap: parse -> struct, tree reused              22947 ns     22.95 µs
clap: build tree only                          306010 ns    306.01 µs

_{238871fba379 vs c10d85cbe1b9 · 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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