docs: freeze the Day 6 analysis methodology

Records D-049 through D-071 before any statistics code exists: the analysis
engine and its neutrality proof, the read-only observer seam, integer time
buckets read from the common message header, fixed-point price and exact
midpoint representation, locked and crossed treatment, time-weighted versus
event-weighted statistics, session boundaries from system events, the
per-second array, the memory-bounded millisecond top-K, per-locate
aggregation, the objective symbol-selection rule, the two-pass workflow,
output scope, weighted percentiles, the absence of trade notional, atomic
outputs, the manifest and checksum policy, independent reconciliation,
finding selection, chart rules, exit codes, and the header split.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: IpsitKalra