Release 0.15.2 with rebranded Observer hero

[danielgwilson]

What changed

• fix the Observer's non-string screenshot placeholder handling so a boolean false cannot render as ../false
• simplify the visible top-left lockup to Humanish while retaining descriptive page-title and accessibility labels
• add a public-safe, rebranded four-lane Observer capture from a real default humanish watch run
• pin the exact PNG checksum in the public binary-asset allowlist
• stage humanish@0.15.2 and make the release test prove the PNG appears in npm's dry-run pack inventory with identical bytes

Capture provenance

• command: pnpm humanish -- watch --port 43179
• run: dryrun-2026-07-14T18-47-37-501Z-1a1256fc
• shape: default four-simulation synthetic run
• verification: all 15 run checks passed; shareSafety.status was share_ready; no warnings
• image: true PNG, 1920 x 550, 300,746 bytes
• SHA-256: 74cd3b6fba5e26fa3a09fec7a886d3af2b4707c7cecbf6e9a997aaa21ef5b6a1

Why the README is unchanged

The new version-pinned unpkg URL cannot exist until 0.15.2 is published. Adding it in this PR would create the exact dead-link interval that #279 prohibits.

After this PR merges and humanish@0.15.2 is published and checksum-verified, a follow-up PR will restore the README hero pinned to:

https://unpkg.com/humanish@0.15.2/docs/assets/humanish-observer-hero.png

That follow-up, not this release phase, will close #279.

Validation

• secret-backed pnpm release:check
• 1,039 tests passed; 10 skipped
• typecheck and build passed
• public-surface scan checked 444 candidate text files and verified 1 binary asset
• skill validation found the single humanish skill
• npm dry-run pack contained 205 files, including the exact 300,746-byte PNG
• focused Observer and release tests passed
• independent visual, release, and adversarial reviews found no remaining blocker

Part of #279.