feat(spec): bind a default when another flag is given

[jdx]

Stacked on #1021 (external_subcommand). Adds clap's default_value_if / default_value_ifs as default_if on the target flag.

Semantics

• Two KDL/attribute arguments are ArgPredicate::IsPresent; three are Equals.
• First matching condition wins.
• Applied only when the target was not on the command line and has no env value.
• Env is applied to every flag before any default_if, so a sibling env can activate the condition regardless of declaration order.
• An applied default_if is a default: it satisfies requires and does not activate requires_if.
• An Equals condition on a bool reads the negate form as "false" (--no-json), matching requires_if.
• clap 4 has the setter and no getter, so the clap bridge leaves default_if empty (same hole as requires).

flag "--bin-names" {
  default_if "--json" "true"
  default_if "--output" "json" "pretty"
}

#[usage(long, default_if("--json", "true"))]
bin_names: bool,

Surfaces

• Spec parse/emit, builder, usage-lib parse
• Derive (default_if / default_ifs) and usage-argv FlagMeta
• Go Meta.DefaultIf, Fill (skips unconditional default when DefaultIf is set), ApplyDefaultIf (takes the parser's negation map)
• Corpus 11-default-if.json (layer: "post-binding")

value_parser ranges remain deferred. Multicall is the next stacked PR.

  

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Instruction counts

benchmark	trend	instructions	Δ	wall (min)	Δ
markdown	▁▁▂▂▃▁▁▂▂█	196,141,583 → 196,953,672	+0.41%	18.22 → 17.57ms	-3.56%
startup	█▄███████▁	829,332 → 825,019	-0.52%	0.84 → 0.83ms	-1.44%

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	4220	—
argh	6292	1.5x
clap	5895248	1396x
bpaf	21917796	5193x

                                              min       p01       p10    median
usage-rs: argv -> struct                      198       201       204       207  ns
argh: argv -> struct                          278       283       292       301  ns
clap: build tree + parse -> struct         508644    512046    515812    521179  ns
bpaf: build parser + parse -> struct      1666119   1666119   1684545   1699706  ns

usage: argv -> struct                             219 ns      0.22 µs
clap: build tree + parse -> struct             506551 ns    506.55 µs
clap: parse -> struct, tree reused              24312 ns     24.31 µs
clap: build tree only                          308074 ns    308.07 µs

_{70eadff1198b vs df69276f650a · measured on the runner, not pushed to the history.}

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