v2: Response capture and HAR export

[loyality7]

Context

v1's Response type explicitly does not capture bodies. Debugging a scrape or an automation often needs to see exactly what came back.

Scope

• Read (and optionally modify) response bodies through the interception path.
• Export a session's full request/response history as a HAR file.

Acceptance criteria

• Response body readable through the existing onResponse/route hooks once implemented for real
• A session can export a valid HAR file that opens in standard HAR viewers
• Body capture respects a size cap — page content is hostile, same posture as script output capping in v1