Write the 0.9.9 changelog

[AllTerrainDeveloper]

Hand-writes the = 0.9.9 = block in readme.txt ahead of the release, so it can ship with ./bin/release.sh 0.9.9 --skip-changelog. No release is cut by this PR and nothing is deployed to WordPress.org.

bin/release.sh drafts this block automatically from the squash-merge titles, but that draft reads like a commit log — "Rebrand (1/2): the OpenStation palette, themes and the mascot", "Make the glow a glow". This is the same twelve PRs rewritten for someone installing the plugin.

What changed against the auto-draft

• The four Mio commits collapse into one line. Mio ships new in this version, so its in-cycle fixes (fix(mio): make the glow a glow #477, fix(mio): wear the brand's own colours, and restore the About scene #478, fix(mio): interpolate the desk between samples, so a dragged window pushes smoothly #481) were never visible to anyone — listing them separately advertises bugs no released build ever had.
• Same for the empty widget column (Restore the widget layer, and make session persistence hold #484), which the rename caused mid-cycle. Kept as a single line because nightly users could have hit it.
• Dropped: the PHPCS chore (Turn PHPCS on for the plugin, not just the extensions #483) and the Playground blueprint rework (Future-proof the OpenStation Playground blueprint #479). Neither is visible to someone installing the plugin.
• Added, missing from the draft: the portal URL moving from /desktop-mode/ to /openstation/ — it breaks installed PWAs, which is the one thing in this release that asks a user to do something — and a closing line summarising the rename for plugin authors.

Two names corrected against their PR bodies

• CPT folders grouped by plugin, WooCommerce integration, and a public tile-drop seam #472 documents the drop seam as wp.desktop.files.registerTilePayloadHandler(), but it merged before the rename. The shipped name is wp.os.files.… — confirmed against src/desktop-files/layer.ts.
• Rebrand (2/2): Desktop Mode becomes OpenStation #475's table says open_station_*() / OPEN_STATION_*. What actually shipped is openstation_* / OPENSTATION_* — confirmed against desktop-mode.php and AGENTS.md.

Notes

npm run build clean; the diff is readme.txt only. The Stable tag: is deliberately untouched — bin/bump-version.sh owns it at release time.

Two things this PR deliberately leaves open, both flagged in the review of #479:

1. The Playground demo links are broken until 0.9.9 ships. blueprint.json and the README button now point at openstation.zip, which no release carries — v0.9.8 only has desktop-mode.zip. The Playground badge, the Studio link and the wp.org Live Preview all 404 in the meantime. Cheapest fix without cutting a release is to upload a copy of v0.9.8's zip to that release under the name openstation.zip; it self-heals once 0.9.9 lands.
2. The rename has no docs/migration-*.md. wp.desktop → wp.os, <wpd-*> → <os-*> and the PHP prefix change are breaking for anyone extending the shell, and AGENTS.md asks for a migration note on a breaking change. Worth writing before 0.9.9 goes out.

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