Repository hygiene: untrack rust/target-fault and ignore Cargo build directories

`rust/target-fault/` is the build directory of the fault-injection candidate
(`cargo build -p own-cli --release --features fault-injection --target-dir
target-fault`, ci.yml). Commit 04c3303 tracked its contents by accident: 223
files and 57 MB of Cargo internals (.fingerprint, .rustc_info.json,
build-script outputs, compiled metadata), present in the qualified candidate
f1d3681 and rightly not a functional failure for the 31/31 qualification run,
which checks the program and not the tree's manners. Build output is not
evidence and is never versioned.

`rust/.gitignore` already ignores `target/`, which is why `rust/target/` never
entered the tree and why `target-fault` did: that pattern does not match a
sibling with a different name. Both directories are now ignored explicitly at
the root.

- every tracked file under rust/target-fault/ is removed
- /rust/target/ and /rust/target-fault/ are ignored in the root .gitignore
- `git ls-files 'rust/target*'` is empty

No functional change, and nothing in the tree read those files: ci.yml builds
the fault-injection candidate into that directory itself. The full
qualification CI runs on this head because the terminal candidate is the SHA
that goes to main, not its almost identical ancestor.

Refs #262

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QfeUtLRm6KFRWcTGQZxQQb

Author: claude

.gitignore

@@ -22,3 +22,13 @@ node_modules/
 # Never commit other projects' code; promote interesting findings into
 # corpus/real-world/ as minimal reduced cases instead.
 corpus/mined/
+
+# Cargo build directories, ignored explicitly at the root. `rust/.gitignore`
+# already ignores `target/`, which is why `rust/target/` never entered the tree
+# and why `rust/target-fault/` did: that pattern does not match a sibling with
+# a different name. `target-fault` is the build directory of the
+# fault-injection candidate (`cargo build -p own-cli --release --features
+# fault-injection --target-dir target-fault`, ci.yml); commit 04c3303 tracked
+# 223 files of it. Build output is not evidence and is never versioned.
+/rust/target/
+/rust/target-fault/