docs: update roadmap status

[davebarnwell]

Summary

Updates the roadmap now that Phases 1 through 3 have shipped.

The document previously still read like the first three phases were pending work. That was no longer accurate after the correctness, ergonomics, typing, portability, and maintenance changes were merged. This PR brings the roadmap back in sync with the actual project state.

User impact

There is no runtime behavior change in this PR. The effect is documentation accuracy.

Maintainers and contributors can now see at a glance which phases are complete, what each completed phase delivered, and what work is still intentionally deferred to the optional backlog. That should reduce duplicate planning and make future issue selection clearer.

Root cause

The roadmap was written as a forward-looking plan and was not updated after the Phase 1, Phase 2, and Phase 3 implementation PRs were merged. As a result, it overstated the amount of remaining work and no longer matched the repository history.

Fix

The roadmap now marks Phases 1 through 3 as completed, adds short outcome summaries for each completed phase, and narrows the remaining plan to Phase 4's optional expansion items.

The suggested issue order and release strategy were also rewritten so they reflect the current state instead of describing already-shipped work.

Validation

This is a documentation-only change, so I did not rerun PHPUnit, PHPStan, or PHP-CS-Fixer.

[Copilot]

Pull request overview

Updates ROADMAP.md to reflect the current project state now that Phases 1–3 have shipped, and reframes Phase 4 as the remaining optional backlog.

Changes:

• Adds a “Current status” section marking Phases 1–3 completed and Phase 4 optional/not started.
• Adds outcome summaries for Phases 1–3 to document what shipped.
• Updates the suggested issue order and release strategy to align with the shipped phases and remaining optional work.

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