docs(README): add hero video — theme-aware cover linking to the 90-second overview

[os-zhuang]

Fixes #11534

Summary

Adds a hero cover to the README, immediately after the chips line
(README.md:22), linking to the official 90-second overview on YouTube
(https://youtu.be/CX_FlOoOtr0).

Revised from the original two-cover plan after maintainer review found
both covers rendering stacked on github.com. Current shape:

• One cover image, dark only — docs/screenshots/hero-cover-dark.png,
  no #gh- fragment on its src. GitHub's rendered HTML (see Verification)
  emits <img> tags with #gh-dark-mode-only / #gh-light-mode-only
  fragments intact but applies no theme CSS to hide either one — so a
  themed pair always showed both, on every renderer, not just mobile.
  hero-cover-light.png is removed (git rm); confirmed via git grep
  that nothing else in the repo referenced it.
• A caption link below the image —
  ▶ Watch: ObjectStack in 90 Seconds — because nothing in a bare <img>
  signals "this is a clickable video" to a reader scrolling past on mobile
  web, desktop, npm's README render, or a plain-text reader; none of those
  surfaces draw a play-button overlay for you.

Assets

docs/screenshots/hero-cover-dark.png was copied from the private
steedos-labs/video-studio repo,
projects/2026-08-objectstack-hero-overview/covers/cover-en-dark.png, and
renamed under docs/screenshots/. Verified byte-for-byte against the
source repo's contents API and by PNG magic-byte inspection before
committing:

file	bytes	magic
docs/screenshots/hero-cover-dark.png	406,703	valid PNG, 2400×1200, 8-bit RGB

hero-cover-light.png (400,524 bytes, same provenance) was added in the
original round and is no longer shipped — removed in this revision as
dead weight once the block stopped referencing it.

Diff shape

README.md diff vs origin/main is a pure 8-line insertion at the
verified anchor (line 22, the chips line) — no pre-existing line touched,
no deletions. docs/screenshots/hero-cover-dark.png is the sole new file
(the light cover, added and then removed within this PR's history, is not
part of the final tree).

Verification

Done:

• docs/screenshots/hero-cover-dark.png present with the byte count above,
  valid PNG magic bytes.
• README diff vs origin/main is a pure insertion — git diff origin/main -- README.md shows 8 insertions(+), 0 deletions(-).
• git grep -n "hero-cover" returns exactly one hit, in README.md,
  pointing at the dark cover with no #gh- fragment.
• Re-rendered via GitHub's own HTML (Accept: application/vnd.github.html on the readme endpoint, ref= this branch)
  after pushing — exactly one hero-cover <img> in the output, src
  carries no #gh- fragment:

  <p align="center" dir="auto">
    <a href="https://youtu.be/CX_FlOoOtr0" rel="nofollow">
      <img src="docs/screenshots/hero-cover-dark.png" width="900" alt="ObjectStack in 90 Seconds — watch the overview on YouTube" style="max-width: 100%;">
    </a>
    <br>
    <a href="https://youtu.be/CX_FlOoOtr0" rel="nofollow"><b>▶  Watch: ObjectStack in 90 Seconds</b></a>
  </p>

• Link and <img>/caption markup are well-formed HTML inside the
  <p align="center"> block.
• grep -naP '[\x00-\x08\x0b\x0c\x0e-\x1f\x7f]' README.md — clean, no raw
  control bytes introduced (the ▶ caption glyph is U+25B6, not a control
  character).

Not done, and not claimed: how the single cover looks to a themed human
viewer (GitHub's rendered visual chrome) — only the structural HTML above
was checked. Left for maintainer eyeball on the rendered PR page.

Gates run

Declared narrowing — verification ran UNLOCKED. scripts/pm/os-verify-lock.sh
could not take the shared verify lock on this host: no usable flock. The shared
verify lock is declared Linux-only (flock is util-linux, and a stock macOS does
not ship it), so the commands below were run directly, without the lock —
a declared narrowing, not a silent one. No serialization guarantee held for
these runs, nor for any sibling agent in this container while they ran.

Gate family re-derived via node scripts/pm/dispatch-gates.mjs against the
actual changed paths (README.md, docs/screenshots/hero-cover-dark.png,
docs/screenshots/hero-cover-light.png) at commit 618c58a56 — same three
as the original round:

gate	verdict
pnpm check:nul-bytes	✅ PASS — "OK (scanned 6650 text file(s) ... no raw ASCII control bytes)"
pnpm check:doc-authoring	✅ PASS — "389 files clean — no bare metadata literals"
pnpm check:doc-anchors	⚠️ NOT MEASURED — exits with "PREREQUISITE NOT MET: github-slugger is not installed" (no node_modules in this docs-only worktree). The gate's own output: "Nothing was measured... NOT a finding." CI will run this on the real farm.
pnpm --filter @objectstack/lint run check:doc-formula-expressions	⚠️ NOT MEASURED — same cause, missing typescript in node_modules. Same caveat as above.

No changeset added (docs-only card; content/docs/releases/ untouched).

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