fix(findings): make --severity filter and list colouring case-insensitive

[ajianaz]

What

• cora findings list --severity <x> now matches regardless of argument case (argument lowercased to match the stored form).
• The list renderer picks red/yellow/green via a new pure severity_color() helper that matches case-insensitively, so colouring actually applies.
• --severity is validated by clap (info|minor|major|critical, any case); unknown values fail with a clear error instead of silently returning nothing.
• The test that pinned the old buggy behaviour now asserts the corrected behaviour.

Why

Severities are stored lowercase, but the filter upper-cased its argument (never matched) and the renderer compared against uppercase literals (colouring dead). Stored data, JSON keys and stats labels are unchanged. Closes #586

Testing

• cargo clippy --all-targets --features tree-sitter -- -D warnings clean; cargo fmt applied.
• cargo test --features tree-sitter --bin cora -- review_store findings: 82 passed (filter for each case variant, unknown value, severity_color, clap parse accept/reject, stats labels still lowercase). index:: / watch:: skipped locally (stray flock); CI runs everything.
• Smoke (debug binary, isolated CODECORA_HOME): --severity MAJOR and --severity major both list the major finding; --severity bogus exits 2 with possible values: info, minor, major, critical; list --json unchanged; forced-colour output shows red/yellow/green for critical/major/minor.

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