fix: clarify TanStack route ignore regex warnings

[BartWaardenburg]

Refs #513

Summary

• clarify diagnostics for TanStack Router routeFileIgnorePattern values that use JavaScript regex syntax unsupported by fallow
• keep default behavior as warn and ignore rather than fail config loading
• add regression coverage from live usage patterns

Verification

• focused fallow-core routeFileIgnorePattern tests: pass
• focused generic plugin invalid-regex tests: pass
• real-project old-vs-new smoke on maximhq/bifrost ui: pass, exits with findings before and after while the warning changes from generic segment-regex wording to routeFileIgnorePattern compatibility wording
• cargo check --workspace: pass
• cargo test --workspace --all-targets: pass
• cargo clippy -p fallow-core --all-targets -- -D warnings: pass
• cargo clippy -p fallow-cli -p fallow-mcp -- -D warnings: pass
• cargo test -p fallow-cli -p fallow-mcp: pass
• cargo fmt --all -- --check: pass
• git diff --check origin/main...HEAD: pass
• typos .: pass
• RUSTDOCFLAGS="-D warnings" cargo doc --workspace --no-deps --document-private-items: pass
• fallow audit --format json --quiet: exits 0 with warn verdict from duplication attribution outside this reviewed diff