validate-lint-format.sh: Rust and Nickel arms ask a different question than their tool

[hyperpolymath]

Summary

.githooks/validate-lint-format.sh gates its Rust arm on whether the repo tracks a Cargo.toml anywhere, then runs cargo from the repo root. cargo asks a different question: is there a Cargo.toml in the current directory or a parent. This repo has none at the root, so the arm fires and then fails for a reason that has nothing to do with the staged code.

This is the recurring estate trap: a guard that asks a different question than its consumer. The comment above the arm (lines 82–86) even names the failure it is trying to prevent — it just prevents it in the wrong repo shape.

Measured evidence

$ ls Cargo.toml
ls: cannot access 'Cargo.toml': No such file or directory

$ git ls-files | grep -c 'Cargo\.toml$'
12

$ cargo fmt --all --check ; echo rc=$?
error: could not find `Cargo.toml` in `<repo root>` or any parent directory
rc=1

$ cargo clippy --all-targets -- -D warnings ; echo rc=$?
rc=101

tracks 'Cargo.toml' '*/Cargo.toml' (.githooks/validate-lint-format.sh:66) returns true because 12 manifests are tracked, so the arm always fires. The consequence: any commit staging a .rs file is rejected, and the message reads Rust: sources are not formatted (cargo fmt --check), which misnames the cause.

There is a second arm with the same shape. The Nickel arm globs *.ncl and feeds every match to nickel, but the .k9.ncl population is mixed:

total .ncl files:                    36
.k9.ncl files:                       16
.k9.ncl starting with `K9!`:          8   # not Nickel source at all

nickel fails those 8 at line 1 column 0 with a parse error on K9!. Extension is not a valid discriminator here; the K9! content marker is.

Acceptance criteria

• The Rust arm runs cargo per crate directory (from each staged file's nearest ancestor Cargo.toml), not once from the repo root.
• With a .rs staged in a crate that is clean, the hook passes; the root having no Cargo.toml is not itself a failure.
• A could not find Cargo.toml condition is reported as a tooling/scope error naming the directory, never as sources are not formatted.
• The Nickel arm skips files whose first bytes are K9!, and says how many it skipped — a denominator, not silence.
• bash -n clean; the arms stay read-only (--check only, no cargo fmt without --check).

Found while measuring #931. Related: the pin-detector defect in #931 is the same trap in a different file.