docs: pin the README example to the release it ships with

[polaz]

Problem

README.md pins the crate at the previous major in its only dependency snippet:

structured-proxy = { version = "3", default-features = false }

The pending release is 4.0.0 (#82 was a breaking change), and docs.rs renders the README as the crate's landing page. Published as is, the first thing a new reader copies resolves to 3.x, which does not have the API the surrounding text describes: that snippet sits in the injected-verifier section, and with_token_verifier is 4.0.0 surface.

The fix belongs in this release cycle, before the release PR (#86) merges. Afterwards it is a 4.0.1 chasing a landing page that was wrong the moment it went up.

Solution

Bump the pin to "4" and re-check the snippet against the API it illustrates, so the example a reader copies matches the version it resolves to.

Acceptance criteria

• No snippet in README.md pins a version older than the one being released.
• The injected-verifier example compiles against the 4.0.0 API surface.
• Merged before chore: release v4.0.0 #86, so 4.0.0 publishes with a correct landing page.

Estimate

20m — one-line change plus verification that the surrounding example still matches the API.