fix(pm): the issue-body footer cell is measured — give each footer shape its own clean class

The header declared the issue-body footer cell UNMEASURED and held the
body-mode append in `mutated` on that ground, while the governed fact
table has carried the measurement since PR #17746 and the skills seat
re-measures it on every seat-post refresh. Since PR #18758 `$?` already
answered 0 for both footer shapes, so the tool said two things about one
set of bytes: "everything sent is on the platform" in `$?` and "MUTATED"
on stderr, with `--json` reporting `body_mutated: true` beside
`body_landed: true`.

`footerReAnchoring`'s two measured shapes are now two declared classes —
`footer-appended` and the new `footer-re-anchored` — for either act, so
`classifyReadBack` takes no `mode`: the comparison is exact bytes, which
cannot tell one act's write from another's, and the gate existed only
for the cell that is now measured. `sentBodyLanded` reads the class
alone. Every exit-4 control is unchanged: a byte lost before the rule, a
byte changed at equal length, a chewed footer, a rewritten link, a
newline from nowhere, a truncation ending in the block.

What is unmeasured is named instead: the WRITE CHANNEL. Two `--body`
refreshes of the same shape on the same endpoint the same day read +58
on this seat's channel and IDENTICAL on the triage seat's, and no act
here can vary that.

Claude-Session: https://claude.ai/code/session_01Gqi43smmqjJ5sUrhfoPeKu
Co-authored-by: Claude <noreply@anthropic.com>

Author: claude