refactor: extract shared filter utilities from existing filters

[bug-ops]

Parent: #717

Problem

CargoBuildFilter contains head/tail truncation logic, ClippyFilter has rule-grouping logic, and TestOutputFilter has failures-only logic. These patterns will be reused by many new filters.

Acceptance Criteria

• Extract truncate_output(lines, max, head, tail) -> (Vec<String>, usize) helper
• Extract strip_noise_lines(output, prefixes) -> (String, usize) helper
• Extract failures_only(output, header_re, summary_re, max, truncate) -> FilterResult helper
• Extract group_by_rule(output, location_re, rule_re) -> FilterResult helper
• Refactor existing filters to use shared utilities
• No behavior changes — existing tests must pass unchanged

[bug-ops]

Resolved by PR #734: migrated to declarative TOML filter engine with built-in rule in default-filters.toml.