fix(coreutils-port): accept localized-Command let-binding in uu_app

[chaliy]

Summary

Yesterday's Coreutils Args Drift workflow on main went red at the regenerating truncate step:

error: unsafe uu_app body: expected a single clap Command builder expression, found 2 statements

Upstream truncate (rev 94b06d97b) now routes the Command through uucore::clap_localization::configure_localized_command(cmd), so the body has the shape:

pub fn uu_app() -> Command {
    let cmd = Command::new("truncate")…;
    uucore::clap_localization::configure_localized_command(cmd)
        .arg(…)…
}

The rewriter elides the localization helper (it pulls in Fluent; bashkit doesn't need it), leaving the body as a 2-stmt let cmd = …; cmd.arg(…).arg(…) form. PR #1617 had tightened the validator to a single expression, which is what this case trips. The on-disk truncate_args.rs was generated before #1617 and already uses this exact shape, so this PR only re-enables the codegen path that produced it.

Fix

Extend validate_uu_app_body in crates/bashkit-coreutils-port/src/args.rs to also accept the 2-stmt shape [Stmt::Local(local), Stmt::Expr(tail, None)] with strict guards so TM-INF-025 does not weaken:

• The let pattern must be a plain Pat::Ident — no mut, no ref, no subpattern, no type ascription, no let … else ….
• The initializer must pass the same per-expression validator (no closures, loops, match, blocks, unsafe, async; only allowlisted macros; only allowlisted method names) AND chain-root at Command::new(...), identical to the single-expression path.
• The tail must pass the per-expression validator AND its receiver chain must bottom out at exactly the binder identifier — not at another Command::new, not at a function call, not at a different variable.

Anything richer (tuple patterns, refutable matches, type ascription, mutability, an extra prefix statement) still fails.

Test plan

• 5 new tests in args::tests cover both directions:
  • positive: accepts_localized_command_let_binding mirroring truncate's exact shape, asserting the wrapper is elided in the emitted body and the tail continues at cmd.arg(…)
  • negative: rejects_let_mut_binding_in_uu_app, rejects_three_statement_uu_app, rejects_tail_rooted_at_other_ident, rejects_let_init_not_rooted_at_command_new
• cargo test -p bashkit-coreutils-port green (37 tests)
• cargo build -p bashkit green
• cargo clippy -p bashkit-coreutils-port --all-targets -- -D warnings clean
• cargo fmt --check clean
• Manually dispatch Coreutils Args Drift after merge and confirm regenerating truncate is green

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Generated by Claude Code

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