refactor(config): add validation for semantic_cache_threshold

[bug-ops]

Summary

Add validation for llm.semantic_cache_threshold config parameter to ensure values are in the valid range [0.0, 1.0] and are finite (rejecting NaN/Inf).

Changes

• crates/zeph-config/src/loader.rs: Added validation check in Config::validate() after env overrides are applied. Validation runs once at bootstrap and rejects invalid values with an actionable error message.
• CHANGELOG.md: Added entry under [Unreleased] section
• Test coverage: 8 new unit tests covering boundary cases (0.0, 0.5, 1.0), out-of-range (-0.1, 1.1), and IEEE 754 special values (NaN, +Inf, -Inf)

Validation Details

• Range: [0.0, 1.0] inclusive (consistent with cosine similarity threshold domain)
• Location: Config::validate() in loader.rs — catches all config load paths (TOML, env override, TUI, CLI)
• Behavior: Rejects (ConfigError) invalid values, does not clamp
• Error message: Includes field name and env var override hint for user guidance

Test Results

• All 6022 tests passing (+8 new)
• No regressions
• Pre-commit checks: ✓ fmt ✓ clippy ✓ nextest

Fixes

Fixes #2036