feat(docs): add recursive help

[jdx]

Summary

• add an explicit portable HelpAll flag action for typed Rust, KDL, usage-lib, and generated Go
• render the selected command and every visible descendant in depth-first display order
• preserve route-specific help and exclude hidden subtrees
• document the action and close the recursive-help PLAN item

Validation

• cargo test -p usage-conformance --test help_request --all-features
• cargo test -p usage-argv --all-features
• cargo test -p usage-lib --all-features
• cargo clippy --workspace --all-features --all-targets -- -D warnings
• cd go && go test ./...

This PR was generated by Codex.

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Note

Medium Risk
Touches parser error variants, help rendering, derive codegen, and Go generation in parallel, so a mismatch in walk order or hidden-command filtering would show up as user-facing help. Behavior is additive and covered by conformance and Go tests.

Overview
Adds a portable ArgAction::HelpAll (action="help_all") that prints long help for the selected command and every visible descendant, in depth-first display_order then name order. Hidden subcommands are skipped; a nested request starts at that command and does not prepend ancestor pages.

The action is wired through typed Rust (Error::HelpAll, render_all / render_all_at), KDL, usage-lib, generated Go (AllHelp, Error.All), the derive/parse() print-and-exit path, and usage-cli. Docs and the PLAN recursive-help item are updated to match.

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[github-actions]

Instruction counts

benchmark	trend	instructions	Δ	wall (min)	Δ
markdown	▄▁▁█	199,315,003 → 218,173,848	+9.46% ⚠️	19.00 → 20.55ms	+8.16%
startup	▁██▇	1,221,718 → 1,221,253	-0.04%	1.35 → 1.37ms	+1.66%

1 benchmark(s) above the 1% gate: markdown +9.46%

_{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	4355	—
argh	6292	1.4x
clap	5895248	1353x
bpaf	21917778	5032x

                                              min       p01       p10    median
usage-rs: argv -> struct                      191       194       196       200  ns
argh: argv -> struct                          265       271       276       288  ns
clap: build tree + parse -> struct         480970    482856    489548    497901  ns
bpaf: build parser + parse -> struct      1557494   1557494   1608581   1654904  ns

usage: argv -> struct                             213 ns      0.21 µs
clap: build tree + parse -> struct             487582 ns    487.58 µs
clap: parse -> struct, tree reused              23767 ns     23.77 µs
clap: build tree only                          304740 ns    304.74 µs

_{d0323d68d2d0 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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