feat(parse): preserve trailing delimiters

[jdx]

Summary

• add an inherited dont_delimit_trailing_values command policy to KDL and typed Rust metadata
• preserve delimited values after -- and on automatic trailing positionals across usage-lib, usage-argv, and generated/direct Go
• carry the clap setting without a fidelity loss and document the portable behavior
• mark the PLAN gap complete and add shared corpus/conformance coverage

Validation

• cargo test --all --all-features
• cd go && go test ./...
• mise run lint
• mise run gen-shadow
• mise run gen-go
• mise run render

This PR was generated by Codex.

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Note

Medium Risk
Changes core argv binding and delimiter handling in Rust and Go across derive, parsers, and generated code; behavior is well covered by tests but mistakes could affect many CLIs that use delimiters or trailing -- forwarding.

Overview
Implements clap's dont_delimit_trailing_values as an inherited, command-wide policy that round-trips through KDL, usage-lib, the typed derive, usage-argv, generated Go, and the clap bridge (no longer a fidelity loss).

Parsing behavior: After -- or once an automatic trailing positional starts, variadic positionals stop splitting on delimiter (e.g. a,b -- c,d → a, b, c,d). Ordinary tokens before that boundary still split. Subcommands inherit the policy from ancestors.

API shift: Event::Arg gains a delimit flag; Rust derive and Go binders honor it when expanding values (and drop the derive's old post-check delimiter split pass). Go also threads Delimiter on flags/args, counts delimited pieces for variadic flags, and uses SplitValue in conformance and generated parsers.

Docs, PLAN, corpus, and conformance tests cover inheritance and KDL round-trip.

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[github-actions]

Instruction counts

benchmark	trend	instructions	Δ	wall (min)	Δ
markdown	██▁	198,671,176 → 195,983,569	-1.35%	18.71 → 18.86ms	+0.80%
startup	▁▁█	1,217,543 → 1,218,362	+0.07%	1.34 → 1.39ms	+4.25%

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	4340	—
argh	6292	1.4x
clap	5893640	1357x
bpaf	21917948	5050x

                                              min       p01       p10    median
usage-rs: argv -> struct                      212       215       218       222  ns
argh: argv -> struct                          269       273       278       286  ns
clap: build tree + parse -> struct         480151    480502    482458    484822  ns
bpaf: build parser + parse -> struct      1573559   1573559   1583532   1606631  ns

usage: argv -> struct                             218 ns      0.22 µs
clap: build tree + parse -> struct             491489 ns    491.49 µs
clap: parse -> struct, tree reused              23413 ns     23.41 µs
clap: build tree only                          301668 ns    301.67 µs

_{616fcfc30619 vs 83ed87bb9e8f · measured on the runner, not pushed to the history.}