docs: align strategy roadmap and policy counts with v0.5.1

[phaethix]

Summary

Aligns the strategy/roadmap docs with what actually shipped in v0.5.1, and
fixes stale eviction-policy counts that still said 10/13 instead of 16.

• docs/design/product-strategy.md:
  • Phase 1 table: mark ✅ E-01 (SIEVE), E-02 (SIEVE-S), E-03
    (AdaptiveClimb), E-05 (benchmark suite).
  • E-04: record that it shipped as the EvictionPolicy enum + pick_victim
    match (not an open trait); re-scoped to Beyond v0.8.
  • E-06: AHE benchmark paper is not written — design differentiator is in
    ahe.md, empirical edge deferred (Pillar 3 "real by design, unproven").
  • Fix quantitative table: 13 → 16 policies; benchmark coverage wording;
    AHE row no longer claims a published paper.
  • 13 → 16 in qualitative + R1; map E-04/E-06 into the Beyond v0.8 backlog.
• README.md: v0.5.1 roadmap line drops the misleading "EvictionPolicy trait"
  claim and adds AdaptiveClimb/AHE; --maxmemory-policy says 16 policies.
• docs/design/market-research.md & docs/guide/configuration.md: 10/13 → 16.

Related

Tracking issue #37 (demonstrate AHE's TTL edge on the opt-in ttl benchmark)
covers the remaining empirical follow-up for E-06.

Test plan

Doc-only change. cargo fmt --check && cargo clippy --all-targets -- -D warnings && cargo test --all-targets should be unaffected.