feat(hud): rate limit format with severity icons (Wave 1-C)

[JeremyDev87]

Problem

Current rate limit badge RL:5h:13%,7d:96% is cramped and non-intuitive:

• Colon-heavy syntax (3 colons)
• Comma as separator is easy to miss
• 96% (critical) looks the same visually as 13% (safe)

Fix (Wave 1-C)

Severity-icon rendering with space separator:

• RL:5h░13% 7d▓96%
• ░ light (U+2591): pct ≤ 60 (healthy)
• ▒ medium (U+2592): 60 < pct ≤ 85 (warning)
• ▓ dark (U+2593): pct > 85 (critical)
• Space replaces comma
• RL: prefix preserved for downstream anchors

Defensive coercion

New _coerce_percentage() helper: None/non-numeric strings default to 0.0 instead of crashing via TypeError (addresses Wave 1-A security reviewer's low finding).

Tests

19 new + 3 updated in test_hud_rate_limits.py:

• All 3 severity buckets
• Boundary checks (60, 85, 85.01)
• Both-tier rendering with mixed severity
• None / non-numeric / empty tier defensive coercion
• Space-separator assertion (no comma)
• Re-export identity lock
• Internal _severity_icon helper exposure test

3 test_hud.py tests updated to new format.

162/162 tests pass.

[JeremyDev87]

Implemented and merged via #1485 (Wave 0-3 integrator) and #1487 (Wave 3b follow-up). Module lives in packages/claude-code-plugin/hooks/lib/. Closing as complete.