fix(lint): clear the clippy backlog the revived Rust CI exposed

[hyperpolymath]

Why main is currently red

PR #358 revived Rust CI. It merged before this commit landed on the branch, so main
now has a working Rust CI that immediately fails its Cargo check + clippy + fmt step.

That is not a regression — it is the gate doing its job for the first time since
2026-06-27. This PR is the backlog it found.

Current state of the revived gate on main (d7f1b49):

success  wasm-tools validate (emitted modules)
success  Cargo build + test (ephapax-cli, --no-default-features)
success  rust-ci / Detect Cargo.toml
failure  rust-ci / Cargo check + clippy + fmt     <-- this PR

What this fixes

cargo clippy --all-targets -- -D warnings failed crate after crate. All resolved;
it now exits 0.

Mechanical, behaviour-preserving:

Lint	Fix
derivable_impls	impl Default for Visibility → #[derive(Default)] + #[default]
manual_try_fold ×2	.fold(Ok(acc), ..) → .try_fold(acc, ..) in the surface and core parsers' statement-sequence desugaring
collapsible_if ×2, match_like_matches_macro	via cargo clippy --fix
ptr_arg	&PathBuf → &Path in two ephapax-cli signatures
unnecessary_unsafe	removed an empty unsafe {} block wrapping only comments in a placeholder test

That last one is worth a second look in a language whose entire thesis is memory safety:
ephapax-runtime::list::tests::test_list_new contained unsafe { /* comments only */ }.

Deliberately allowed, each with an inline reason (no blanket crate-level allow):

• approx_constant — lex("3.14") is a float literal under test, not an approximation of PI
• type_complexity — replaced with a TokenPredicate type alias
• too_many_arguments ×3 — 8 params vs clippy's default threshold of 7, on option-heavy CLI
  entry points and the recursive import resolver
• large_enum_variant — Value::Closure is ~264 bytes vs ~48 for the next largest, so every
  Value pays for it. The real fix is boxing the payload, which touches every construction
  and match site in the interpreter. That is a behavioural-risk refactor that does not
  belong in a CI-infrastructure change. Allowed here with a comment, tracked in clippy: box the Value::Closure payload (large_enum_variant, currently allowed) #360.

Verification — all local, all green

cargo check --locked --all-targets          ok
cargo clippy --all-targets -- -D warnings   exit 0
cargo fmt --all -- --check                  clean
cargo test --workspace                      285 passed, 0 failed

The 285 passing tests matter here: the try_fold rewrites and the unsafe removal are the
only changes that could alter behaviour, and the parser/interpreter suites cover both.

🤖 Generated with Claude Code