docs: unred main -- two "mirror" false positives tripped the cutover ratchet

[wshallwshall]

main is red, and this unreds it

test_present_tense_mirror_prose_does_not_grow counts 54 against a ceiling of 52 on main.
Every open PR inherits the failure the moment it merges main -- two already have.

Measured, not inferred. Counting the module's own predicate over the module's own file
population (_tracked(), which excludes _HISTORICAL and the test itself):

ref	hits	ceiling
40150e04 (parent)	52	52	ok
d059ddf6 (main)	54	52	over

The diff between those two sets is exactly two lines, both added by the previous merge.

Both are false positives, of a kind this module already documents

_PROSE matches the mirror. Both new lines use the word in its ordinary English sense -- a
symmetric counterpart to a rule -- not the repository sense the ratchet exists to catch.
Neither claims this repository is a mirror of anything:

• CLAUDE.md -- "The mirror of the rule above, and the more insidious half"
• docs/LEDGER-GATE.md -- "The allocation rule has a mirror, and the mirror is the more insidious half"

Reworded to counterpart, which is what both sentences already say. Meaning unchanged in each.

The ceiling is deliberately not raised

The module's comment is explicit -- lower it when you fix some, never raise it -- and it
records that 54 -> 52 was taken rather than banked so the slack could not re-admit real rot.
Raising it back to 54 to accommodate two non-defects would hand exactly that credit back, and
test_the_ratchet_is_not_slack exists to prevent precisely that.

Verified both directions

A green gate is evidence only if it was shown it could still go red, so both states were run:

• with the two lines reverted -- assert 54 <= 52 fails
• with them reworded -- passes

Full module: 4 passed.

Not fixed here

_PROSE will keep firing on innocent uses of "the mirror", which is a common way to describe a
symmetric rule. This is the second false-positive class the pattern has produced, after the
private repo substring case the module already narrowed for. Narrowing it deserves the same
careful verification that change got, and does not belong in a commit whose job is to unred main.