fix(file-saver): block symlink escapes on save

[chaliy]

What

Harden LocalFileSaver against symlink-based escapes under base_dir.

Closes #97.

Why

Lexical normalization blocked .. traversal, but a pre-existing symlink inside the save root could still redirect writes outside the allowed directory. execute_with_saver() also split validation and write into separate steps, which left a race window.

How

• walk each parent directory at save time under the canonical base directory
• reject symlinked parent components and symlink final targets before writing
• keep the canonicalized path pinned under the canonical base directory
• remove the separate validate_path() preflight from execute_with_saver() so checks happen at write time
• add regression coverage for direct saver writes and execute_with_saver() symlink escapes
• update the threat model to mark this path as mitigated

Risk

• Low
• Touches only file-save path handling and adds regression tests around the changed behavior

Checklist

• Unit tests are passed
• Smoke tests are passed
• Documentation is updated
• Specs are up to date and not in conflict
• cargo fmt --all is passed
• cargo clippy --workspace --all-targets -- -D warnings is passed
• RUSTDOCFLAGS="-D warnings" cargo doc --workspace --no-deps is passed
• cargo build --workspace --exclude fetchkit-python --release is passed