feat: build remaining observability deck views

[Trecek]

The observability deck previously left context, errors, trend, gaps, and harness/provider parity as planned routes. This change builds all five as offline views backed by the derived report index and shared Python aggregation.

• Retain per-turn usage and coverage, refresh the index when descriptor/ledger evidence changes, and preserve exact session ownership. The derived index schema advances to v3 and rebuilds automatically.
• Show cache-read occupancy against each turn’s resolved model window, cache read/write/output series, and an inclusive-input toggle that starts off. Measure linked parent prompts and visible subagent returns with the parent-model tokenizer and explicit provenance.
• Expose error symptoms with skill/step attribution, time-window comparisons, an evidence-driven gap register, and source-pair field coverage. Unsupported or absent observations remain distinct from measured zero.
• Wire the new populations through CLI generation, filtering and navigation; package embedded assets and document the evidence contracts.

Validation:

• All-file pre-commit passed.
• Configured MCP gate (task test-local-gate) passed: 50,990 passed, 612 skipped, 29 expected failures; lint, import contracts, and Linux/Darwin type checks passed.
• Seeded CLI generation succeeded. Chromium 145 completed 34 checks across new and existing routes, light/dark themes, input toggling, missing versus measured-zero values, selection links, and offline operation without page errors or runtime network requests.
• PDF content-stream inspection confirmed vector chart output without raster chart images, including backgrounds enabled/disabled, dark screen preference, and forced-color scenarios.

Closes #4652