feat(parse): add argument subcommand conflicts

[jdx]

Summary

• add args_conflicts_with_subcommands across KDL, typed Rust, usage-lib, the clap bridge, generated Go, and Go runtime lowering
• reject selecting a child after its parent has bound a flag or positional while leaving child-owned arguments unchanged
• document the policy and remove its obsolete clap fidelity warning

Verification

• cargo test --all --all-features
• cargo clippy --all --all-features -- -D warnings
• go test ./... (from go/)

AI-generated pull request.

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Note

Medium Risk
Core argv routing changes in Rust and Go parsers, but the behavior is opt-in per command and covered by cross-layer tests; wrong enforcement would mainly affect CLIs that enable the flag.

Overview
Implements args_conflicts_with_subcommands, matching clap: when enabled on a command, binding any parent flag or positional makes a later child subcommand name an error; subcommands alone still work.

The change threads a new command flag through KDL/spec, usage-lib (including default-subcommand routing), usage-argv and Go parsers (command_arg_found / CodeSubcommandConflict), derive (#[command(args_conflicts_with_subcommands)]), generated Go, clap bridge, and styled diagnostics. clap_usage no longer reports this setting as a fidelity loss.

PLAN.md and migration/compatibility docs mark the policy as portable; tests cover Rust argv, usage-lib, Go, facade derive, and clap bridge preservation.

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[cursor]

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

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[github-actions]

Instruction counts

Nothing was compared, and so nothing was gated. No series appears on both sides: either the base has no measurements recorded, or the two were measured on different runner classes, which are deliberately not comparable — counts shift between machine types by more than a real regression does.

New, nothing to compare against: markdown on bamboo-v2-ubuntu24.04-x64-30vcpu-24gb-rust1.97.1, startup on bamboo-v2-ubuntu24.04-x64-30vcpu-24gb-rust1.97.1

_{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	4265	—
argh	6292	1.5x
clap	5893652	1381x
bpaf	21917954	5139x

                                              min       p01       p10    median
usage-rs: argv -> struct                      188       191       193       198  ns
argh: argv -> struct                          280       286       293       303  ns
clap: build tree + parse -> struct         480560    481890    484601    493720  ns
bpaf: build parser + parse -> struct      1620140   1620140   1627450   1655290  ns

usage: argv -> struct                             197 ns      0.20 µs
clap: build tree + parse -> struct             485161 ns    485.16 µs
clap: parse -> struct, tree reused              23428 ns     23.43 µs
clap: build tree only                          300856 ns    300.86 µs

_{adb30558cc01 vs 1d8566ea1393 · measured on the runner, not pushed to the history.}