feat(tui): add compaction probe metrics to TUI dashboard (#2049)

[bug-ops]

Summary

Display compaction probe validation statistics in the TUI memory panel for post-compression context integrity monitoring.

Features:

• Probe pass rate, soft/hard failure rates, error count (rate distribution line)
• Last probe verdict (Pass/SoftFail/HardFail/Error) with score (color-coded line)
• Conditional rendering in memory panel (hidden when no probes run)
• Color-coded verdict: Green (Pass), Yellow (SoftFail), Red (HardFail), Gray (Error)

Changes

Core

• zeph-memory: Add ProbeVerdict::Error variant (metrics-only, not used by probe logic)
• zeph-core/metrics.rs: Add last_probe_verdict: Option<ProbeVerdict> and last_probe_score: Option<f32> fields
• zeph-core/summarization.rs: Update 4 update_metrics() calls to set new fields; handle Error variant

TUI

• zeph-tui/widgets/memory.rs: Render probe stats in two-line layout with color-coding and abbreviations (P/S/H/E)
• Add 3 snapshot tests (Pass, SoftFail/HardFail, Error verdicts; zero-probes edge case)

Design Decisions

Per approved architecture (issue #2049 design review):

1. Flat fields on MetricsSnapshot (not substruct) — consistent with existing metrics pattern
2. Render in Memory panel — semantically belongs with memory compaction metrics
3. Two-line conditional display — fits narrow TUI side panel
4. Color-coded enum with exhaustive match — compile-time safety vs. string-based matching
5. Error verdict clears score (Option<f32> = None) — prevents stale/misleading data

Validation

• ✅ Architect: approved (design v2 addressing critic feedback)
• ✅ Critic: approved (3 important issues resolved)
• ✅ Developer: all tests pass (6065/6065), clippy clean, fmt check pass
• ✅ Tester: test coverage complete (all 9 AC verified)
• ✅ Perf: +16 bytes to MetricsSnapshot, negligible impact
• ✅ Security: low risk, no information disclosure
• ✅ Impl-critic: code quality clean, 1 important finding addressed (debug_assert canary)
• ✅ Reviewer: approved for merge after debug_assert fix applied

Closes #2049