chore: migrate to Rust edition 2024

Bump edition 2021 -> 2024. rust-version stays at 1.91, already well above
the 1.85 floor the edition needs; docs/guide/resources/troubleshooting.md
still claimed 1.70+, which is where a failed `cargo install --git` lands.

Four things the edition forces:

- std::env::{set_var,remove_var} are unsafe in 2024 with no safe std
  replacement. Rather than wrap the test call sites in unsafe -- which the
  crate denies and .semgrep.yml flags -- route them through temp-env, a
  dev-only dependency whose closure API is safe and which restores the
  previous value even when the body panics. The hand-rolled CLAUDE_DIR_LOCK
  and PI_DIR_LOCK guards existed only to serialise those mutations and are
  now redundant; CWD_LOCK and TEST_ENV_LOCK stay, they order more than the
  env var itself.

- unsafe_op_in_unsafe_fn is on by default, so the libc calls in the proxy
  signal handler and in stream.rs's relay handler need explicit unsafe
  blocks, scoped to the libc calls themselves.

- `gen` is a reserved keyword, so the closure by that name in diff_cmd.rs
  becomes make_lines.

- Tightened tail-expression temporary scopes let clippy prove the binding in
  setup_test_env is inlinable, so let_and_return now fires there.

if_let_rescope changes when the scrutinee temporary drops in an if let/else.
The two sites in show_claude_config take cargo fix --edition's match rewrite,
which keeps the 2021 drop timing.

rustfmt.toml is kept rather than dropped: cargo fmt passes --edition from
Cargo.toml, but a bare rustfmt invocation has no crate context and falls
back to edition 2015, which cannot parse the let-chains the next commit
introduces. Pinning it there keeps format-on-save and pre-commit hooks in
agreement with CI.

clippy::collapsible_if is allowed crate-wide for now; the follow-up commit
adopts let-chains and removes the allow.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Cargo.lock

@@ -723,6 +723,15 @@ version = "0.8.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "6373607a59f0be73a39b6fe456b8192fcc3585f602af20751600e974dd455e77"
 
+[[package]]
+name = "lock_api"
+version = "0.4.14"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "224399e74b87b5f3557511d98dff8b14089b3dadafcab6bb93eab67d3aace965"
+dependencies = [
+ "scopeguard",
+]
+
 [[package]]
 name = "log"
 version = "0.4.29"
@@ -781,6 +790,29 @@ version = "0.2.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "04744f49eae99ab78e0d5c0b603ab218f515ea8cfe5a456d7629ad883a3b6e7d"
 
+[[package]]
+name = "parking_lot"
+version = "0.12.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "93857453250e3077bd71ff98b6a65ea6621a19bb0f559a85248955ac12c45a1a"
+dependencies = [
+ "lock_api",
+ "parking_lot_core",
+]
+
+[[package]]
+name = "parking_lot_core"
+version = "0.9.12"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2621685985a2ebf1c516881c026032ac7deafcda1a2c9b7850dc81e3dfcb64c1"
+dependencies = [
+ "cfg-if",
+ "libc",
+ "redox_syscall",
+ "smallvec",
+ "windows-link",
+]
+
 [[package]]
 name = "percent-encoding"
 version = "2.3.2"
@@ -863,6 +895,15 @@ version = "6.0.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f8dcc9c7d52a811697d2151c701e0d08956f92b0e24136cf4cf27b57a6a0d9bf"
 
+[[package]]
+name = "redox_syscall"
+version = "0.5.18"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ed2bf2547551a7053d6fdfafda3f938979645c44812fbfcda098faae3f1a362d"
+dependencies = [
+ "bitflags",
+]
+
 [[package]]
 name = "redox_users"
 version = "0.4.6"
@@ -939,6 +980,7 @@ dependencies = [
  "serde",
  "serde_json",
  "sha2",
+ "temp-env",
  "tempfile",
  "toml",
  "toml_edit",
@@ -1025,6 +1067,12 @@ dependencies = [
  "winapi-util",
 ]
 
+[[package]]
+name = "scopeguard"
+version = "1.2.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "94143f37725109f92c262ed2cf5e59bce7498c01bcc1502d7b9afe439a4e9f49"
+
 [[package]]
 name = "semver"
 version = "1.0.27"
@@ -1159,6 +1207,15 @@ dependencies = [
  "syn",
 ]
 
+[[package]]
+name = "temp-env"
+version = "0.3.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "96374855068f47402c3121c6eed88d29cb1de8f3ab27090e273e420bdabcf050"
+dependencies = [
+ "parking_lot",
+]
+
 [[package]]
 name = "tempfile"
 version = "3.26.0"