[Blueprints] Unify website/CLI version-override merging

[fellyph]

Summary

Extracts the duplicated "blueprint preferredVersions override" merge into a single shared helper, mergeBlueprintVersions, in @wp-playground/blueprints, and has both the website and the CLI delegate to it. Removes the long-standing TODO in blueprints-v1-handler.ts that flagged the duplication.

Precedence is unchanged in principle — external override (URL query param or CLI flag) > blueprint's preferredVersions > hardcoded default — but the CLI path had real bugs that the consolidation fixes.

Changes by commit

• 6e397f15e [Blueprints] Add mergeBlueprintVersions helper — new pure function in packages/playground/blueprints/src/lib/merge-blueprint-versions.ts. Returns a new blueprint declaration without mutating the input. Short-circuits when preferredVersions.wp === false so the PHP-only opt-out is honored.
• d4ca4142f [Blueprints] Export mergeBlueprintVersions from package entry — re-exports the helper and the BlueprintVersionOverrides type so the website and CLI can consume them through the package's public surface.
• 830e8e141 [Blueprints] Add tests for mergeBlueprintVersions — 9 unit tests covering defaults, blueprint-only values, overrides winning over blueprint values, partial overrides (wp only / php only), empty-string overrides falling through, undefined overrides falling through, wp === false short-circuit, input immutability, and preservation of unrelated blueprint fields.
• 8d4a0643d [CLI] Use shared mergeBlueprintVersions in getEffectiveBlueprint — replaces the local merge in packages/playground/cli/src/blueprints-v1/blueprints-v1-handler.ts. See Breaking changes below.
• b006e67fc [Website] Use shared mergeBlueprintVersions in applyQueryOverrides — replaces the local merge in packages/playground/website/src/lib/state/url/resolve-blueprint-from-url.ts. The 6.3 WP_DEVELOPMENT_MODE workaround still runs at the call site, after the merge. Drops the now-unused RecommendedPHPVersion import.

Breaking changes (CLI only)

Three user-visible behavior changes for the CLI; the website is unchanged. Per AGENTS.md, surfacing here:

1. --wp and --php now actually win over the blueprint. The previous CLI code spread resolvedBlueprint?.preferredVersions at the end of the preferredVersions object, which silently undid the flag override whenever the blueprint declared its own version. After this change, the documented precedence (flag > blueprint > default) is what runs.
2. Empty-string flags no longer set the version to ''. The previous CLI code used ??, so --php= was treated as a real override of ''. Now it falls through to the blueprint or default (the website's prior behavior).
3. preferredVersions.wp === false is now honored by the CLI. A blueprint declaring wp: false (PHP-only opt-out) used to be ignored by the CLI, so --wp=6.8 would inject wp: '6.8' and risk tripping the compile-time guard that rejects WP-only fields when wp === false. The CLI now short-circuits in this case, matching the website.

Test plan

• npx nx test playground-blueprints (188 tests, including the 9 new ones)
• npx nx run-many --target=typecheck --projects=playground-blueprints,playground-cli,playground-website
• npx nx run-many --target=lint --projects=playground-blueprints,playground-cli,playground-website
• CLI smoke: npx nx dev playground-cli server --wp=6.8 --php=8.4 --blueprint=<a blueprint with conflicting preferredVersions> and confirm flags win
• CLI smoke: blueprint with preferredVersions: { wp: false, php: '8.3' } plus --wp=6.8 and confirm wp is not injected
• Website smoke: load ?wp=6.8&php=8.4 against a blueprint with different preferredVersions and confirm the URL wins
• Website smoke: confirm ?wp=6.3 still applies the WP_DEVELOPMENT_MODE workaround

Out of scope

The earlier investigation surfaced three further consolidation opportunities that are not in this PR and could be done as follow-ups:

• Validate ?php=next on the website the same way the CLI's yargs choices does.
• Unify WordPress version resolution (CLI dynamically queries wordpress.org; website uses a hardcoded switch in a generated file).
• Document supported aliases (beta, trunk, nightly, custom URLs, version-prefix matching) consistently on both sides.

[mho22]

LGTAI. It also suggested to create a test that pins down the new "flag wins over blueprint" precedence.

It looks also good to me too of course.