extension: remove startup PlayHTML capability scan

[spencerc99]

Summary

• stop scanning every page for PlayHTML capability attributes during content-script initialization
• remove the now-unreachable startup scan method
• preserve the on-demand CHECK_PLAYHTML_STATUS path and its live DOM count
• add executable content-entrypoint coverage that proves startup skips the selector while an explicit status request sees elements added after initialization

Rationale

The startup result was discarded, so every supported page paid for a full capability-selector query without changing extension behavior. The popup status request still needs a current count and continues to scan on demand.

Verification

• bun run build-packages
• focused regression test, including a pre-fix failure on the startup selector
• bun run test:extension: 141 files and 897 tests passed
• bun run check:extension
• production Chrome MV3 build
• bun run smoke:extension: service worker, content script, and popup loaded in isolated headless Chrome
• git diff --check
• final conservative diff review: no findings

Scope and overlap

PR #422 was checked and is closed without this cleanup. Current open extension PRs were checked; #264 also touches content.ts for an unrelated auth challenge bridge and does not duplicate this removal.

No screenshots because visible UI behavior is unchanged. No extension/PENDING.md entry because this is internal startup maintenance. No package docs, starter templates, or changeset are needed because published package behavior and APIs are unchanged.

[pkg-pr-new]

Open in StackBlitz

bun add https://pkg.pr.new/@playhtml/common@427

bun add https://pkg.pr.new/playhtml@427

bun add https://pkg.pr.new/@playhtml/react@427

commit: 4b5dd5f

[cloudflare-workers-and-pages]

Deploying we-were-online-website with    Cloudflare Pages

Latest commit:	4b5dd5f
Status:	✅  Deploy successful!
Preview URL:	https://d3b53462.we-were-online-website.pages.dev
Branch Preview URL:	https://cx-remove-startup-playhtml-s.we-were-online-website.pages.dev

View logs

[spencerc99]

Code review

No issues found. Checked for bugs and project-instruction compliance.