Body match-and-replace rules

Extends match-and-replace rules to request/response bodies, not just
headers. A body rule materializes the body into memory (bounded by
the same maxCaptureBytes cap as history capture) instead of streaming
it straight through - the opposite of the normal path, so it's only
paid when a body rule is actually configured. A body over the cap
passes through byte-exact and unmodified rather than being partially
rewritten.

Response Content-Length is recomputed explicitly when a rule changes
body length: unlike http.Request.Write, http.ResponseWriter doesn't
derive it from resp.ContentLength on its own, so a stale header would
otherwise corrupt response framing for the client.

The history audit trail still shows the original, pre-rule bytes on
both legs; only the wire traffic reflects the rewrite. Verified live
against a real daemon: origin receives the rewritten request body,
client receives the rewritten response body with correct
Content-Length, and history keeps the unmodified bytes.

Adds a Part selector (header/body) to the Rules add/edit form and
table in the TUI.

0 files changed, 0 insertions, 0 deletions