Fix mixed cumulative overcounting and release v2.6.3

[zJay26]

Mixed cumulative records in v2.6.0–v2.6.2 could recount a whole earlier turn after one reset. For example, total/last values of 100/100 → 20/20 → 50/30 produced 170 instead of 150. Evaluate each turn boundary independently, preserve continuing and duplicate baselines across scan restarts, and report incomplete boundary evidence conservatively.

Schema v10 preserves existing v9 ledgers and cursors while immediately showing a repair notice and pausing incremental ingestion until the user explicitly rebuilds retained JSONL. Regression tests cover the migration, Dashboard confirmation API, mixed counters, corrections, duplicates, and missing snapshots. Update both READMEs, accounting documentation, release notes, and the version to 2.6.3.

Validation:

• Go 1.26.8: go test ./... and go vet ./... passed.
• Dashboard and demo: all 23 Playwright tests passed.
• An isolated accounting-only projection of 766 frozen rollouts matched all 742 per-session reference totals after rebuilding, removing 4,704,311,574 confirmed repeated tokens across 23 sessions. Changed sessions were independently reconciled against source records; unchanged sessions retained snapshot reference totals. Repeating the scan inserted zero events and left the total unchanged. Private source records and audit databases remain local.
• Checked documentation links, unchanged pricing tables, and git diff --check.