docs(requirements): add contextual user model requirements with scoring model rationale

Add requirements.md with outcome-focused EARS criteria for the unified
contextual user model. Covers 10 functional requirements (FR-0 through
FR-9) spanning algorithm documentation, dynamic spans, walk strategy,
structural fixes, KN backoff model, walk integration, KeyHandler wiring,
legacy removal, migration, and technical debt.

FR-0 requires algorithm.md to document both the Viterbi DP walk and
the scoring model rationale: legacy score hack problem, seven-technique
smoothing comparison (Absolute Discounting, KN variants, Bayesian,
Witten-Bell, Jelinek-Mercer, Stupid Backoff), the "丼 problem" design
breakthrough, and four-level backoff formulas.

All criteria use EARS syntax with no C++ implementation details in
required sections. Context blocks provide design rationale, pseudocode,
and academic references (SLP3, FSNLP, Chen & Goodman 1999).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: tianjianjiang