Roadmap: consumer gap analysis + Wave 4-6 plan

[rasuvaeff]

Docs-only. Adds a consumer-perspective gap analysis to ROADMAP.md after the 2.1.0 release, comparing the surface against Hypothesis, fast-check, jqwik, Hedgehog and Eris.

Changes

• New "Consumer gap analysis vs. analogs (2026-07-05)" section with a feature-comparison matrix.
• Wave 4 → 2.2.0: stateful / model-based testing (the single biggest gap; no PHP peer has it).
• Wave 5 → 2.3.0: explicit examples + persist-and-replay of the minimal counterexample.
• Wave 6 → 2.4.0: domain arbitraries + regex/stringMatching, in-body Gen::draw().
• Backlog (Tier 3): target(), function generation (CoArbitrary), per-example deadline.
• Header version bumped 1.0.0 → 2.1.0; Status section updated (2.1.0 done via PR 2.1.0: coverage gate, charset/bytes/enum/unique/interval/recursive generators, shrink debugging #6).
• "No stateful testing" moved out of Non-goals (promoted to Wave 4), with a note recording the reversal.

All planned items are additive — none touch the ArbitraryInterface contract or existing seed sequences, so all ship as minors.

No code changes.