log_ring: seqlock the records to prevent torn cross-process reads

The lossy MPSC log ring published each record by writing its text and THEN storing
the slot's sequence. A consumer that passed the seq==generation check could then
read text while a producer 'capacity' generations later overwrote that same slot
(it wrote text before bumping seq), yielding a torn line. Diagnostics-only and
practically unreachable (it needs the consumer a full ring behind -- ~60k lines/s
between two host drains), but a real data race.

Make it a proper seqlock: the producer stores seq 0 (in-progress) and fences
BEFORE touching the record, then publishes the generation after the text; the
consumer copies the record out and re-checks seq, dropping the line if it changed.
The ring stays lossy, never torn.

Adds log_ring_test (previously zero coverage): a deterministic wrap-drop case plus
a threaded torn-read guard (4 producers + a slow consumer on a 32-slot ring) that
emits 0 torn lines out of ~300k produced. Closes both the cross-process item and
the log_ring coverage gap.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -113,11 +113,7 @@ default** and covers anything the hooked path doesn't.
 From an in-depth review pass. Each item is fixed test-first (a failing test, then the fix) and lands
 as its own commit; "verify" items are confirmed real before any change, and dropped if not.
 
-Cross-process / ABI:
-- **`log_ring` torn-text window** — verify the MPSC overwrite race; fix or bound it.
-
 Test coverage:
-- **`log_ring`** — threaded push/drain + wrap-skip generation test (currently zero coverage).
 - **Seqlock reader paths** — torn-read retry, odd-sequence skip, attempt-exhaustion → false.
 - **Version/magic mismatch rejection** — negative test that the hook refuses a bad version.
 - **Dedicated hook tests** — `focus_spoof`, `vk_hook` (present), `d3d9_hook`.