px-2ms: refuse \U like \u in string escapes

[johnnyt]

Resolves px-2ms (campaign RF047, RQ-RF047-8: refuse \U, the document-only arm declined).

What changed

The lexer's escape refusal in take_string/6 matched lowercase ?u only, so
"caf\U00e9" fell through the generic escape clause and lexed silently to
"cafU00e9". The clause now guards on u in [?u, ?U] and returns the same
ParseError message for both spellings. This is a narrowing of the existing
refusal - exactly the surface the ruling names - and nothing else about escapes
changes: "a\qb" is still aqb and "Unknown \x escape" is still
"Unknown x escape".

• lib/predicator/lexer.ex - the guard and the comment above it.
• test/predicator/lexer_test.exs - one negative test pinning \U refused with
  the byte-identical \u message, with a one-line sabotage note above it.
• docs/reference/language.md - the "any other escaped character stands for
  itself" rule now names \u and \U as the refused spellings (the narrowing
  the change makes to that rule), and the refusal example carries a doctest for
  each spelling. Both run under test/docs_examples_test.exs.
• changelog.d/px-2ms.md - a ### Fixed fragment (px changelog.d/README.md
  :22-27: "a bug fix a user could have noticed").

No version bump (RQ-RF046-13 stands: no predicator 9.4.2; the fix ships with the
next release). @version is unchanged at 9.4.1.

Sabotage

Mutation: drop ?U from the when u in [?u, ?U] guard. Run red - the new test
fails with {:ok, [{:string, 1, 1, 8, "U0041", :double, {1, 9}}, ...]} instead of
an error. Reverted from a copy taken before the mutation; mix test test/predicator/lexer_test.exs green after the revert. (px's
gate.sabotage.test_roots is a fixed file list that excludes lexer_test.exs,
so the gate does not demand the note; the bead does, so it is written.)

Gate

mix quality on the committed tree, green:

✓ Format: No changes needed (357ms)
✓ Compile: dev + test compiled (warnings as errors) (884ms)
✓ Dependencies: No unused dependencies (390ms)
✓ Credo: No issues (3.8s)
✓ Tests: 2,914 of 2,914 passed, 95.6% coverage (4.8s)
✓ Dialyzer: No warnings (PLT built this run) (49.3s)
✓ All quality checks passed!

gate=green. The repo gate lock and a machine slot were held across the gate run
and the commit; the committed tree is byte-identical to the tree that ran green
(git write-tree = 5e715dabd811de82c520a8feea96a428d6a1c31f before and after
git commit), so the commit step's internal gate re-run was taken in the
bare-commit shape citing this run.

Firewall

Terminology scan over the full outbound diff plus the new fragment: hard=0,
soft=0. Constructed positive control: one term from the firewall's hard pattern
(the vendor/integration-name arm) injected into a throwaway copy of the scanned
text; the scan printed a match on that copy, and the copy was deleted when the
result was read.

Provenance

Files touched are exactly the brief's file map minus conformance/:
lib/predicator/lexer.ex, test/predicator/lexer_test.exs,
changelog.d/px-2ms.md, docs/reference/language.md. conformance/ carries no
\u or \U case (grep over conformance/ returns zero hits), so the also-gated
path is untouched. No sibling PR was rebased over; the branch is cut from
origin/main at d06ece1 and is one commit ahead. docs/adr/ is untouched
(git diff --stat origin/main -- docs/adr/ is empty).

Scout finding F4 is addressed: the general rule at docs/reference/language.md
that the \q doctest pins is restated so it stays true under the narrowing, and
the \q and \x siblings stay green.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
lib/predicator/lexer.ex	97.86% <100.00%> (+<0.01%)	⬆️

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