docs(tt4,nfr): record Tier-4 + NFR verification (ADR-0008 A1, ADR-0057)

Document the regression-hardening work from this session:

- ADR-0008 Amendment 1 — Tier 4 realized: the portable-pty + vt100 harness,
  expectrl dropped, serial/fail-fast design, the four flows, sidebar-region
  reads, command pacing (issue #39), CI default-target (advances TT5).
- ADR-0057 (new) — NFR verification strategy: contrast + ΔE2000 gates
  (NFR-5/7), startup/RSS measured (NFR-1/3: release ~29 ms / ~10 MB),
  NFR-2 by architecture, NFR-4/6 reviewer note. Records the two shipped
  light-theme contrast defects this work caught and fixed.
- requirements.md: TT4 [~]->[x], TT5 [/] (only a Windows exec runner left),
  NFR-1/2/3/5/7 ->[x], NFR-4/6 ->[/], each with evidence.
- CHANGELOG.md (new): Keep a Changelog + SemVer, [Unreleased] + [0.2.0].
- README ADR index updated; handoff-76; the working plan.

Docs-only (CI gate skips via paths-ignore).

CHANGELOG.md

@@ -0,0 +1,53 @@
+# Changelog
+
+All notable, user-facing changes to RDBMS Playground are documented here.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0/).
+
+## [Unreleased]
+
+### Added
+- Install via **Scoop**, **Homebrew**, and **winget** in addition to the
+  existing channels.
+- Tier-4 end-to-end test suite that exercises the real application in a
+  terminal (cold launch, save/reopen, export/import, undo) — guards against
+  regressions the unit and render tests can't see.
+- Automated colour-accessibility checks: every theme now has its
+  foreground/background contrast and its syntax-colour distinctness verified
+  on each build.
+
+### Fixed
+- **Light theme:** string-literal and flag colours in syntax highlighting
+  were below the WCAG-AA contrast bar; both are now legible. Two dark-theme
+  token colours that were hard to tell apart have been separated.
+- Windows `install.ps1` now works on the in-box Windows PowerShell 5.1 and
+  updates `PATH` for the current session.
+
+## [0.2.0] - 2026-06-17
+
+### Added
+- **Version surfaces:** a `--version` / `-V` command-line flag and an in-app
+  `version` command, both reporting the build's exact version.
+- **Installation options:** publish to crates.io (`cargo install
+  rdbms-playground`), `cargo binstall` support, a one-line `curl | sh`
+  installer, and a Windows `install.ps1` — so you no longer have to hand-pick
+  a release asset.
+- **Documentation & landing site** at <https://relplay.org> — the canonical
+  user guide plus screencast demos.
+- A demonstration-mode key alias (Ctrl-G acting as F1) so screencasts can
+  show the in-app help/hint key.
+
+### Fixed
+- Corrected several errors in the contextual-hint help text.
+
+## [0.1.0] - 2026-06-15
+
+First public release: the cross-platform terminal sandbox for learning
+relational-database concepts — tables, keys, relationships, indexes, queries
+and query plans — in a guided simple mode or a full advanced (SQL) mode, with
+projects, undo, and export/import.
+
+[Unreleased]: https://git.lazyeval.net/oli/rdbms-playground/compare/v0.2.0...HEAD
+[0.2.0]: https://git.lazyeval.net/oli/rdbms-playground/compare/v0.1.0...v0.2.0
+[0.1.0]: https://git.lazyeval.net/oli/rdbms-playground/releases/tag/v0.1.0