Check the digest before paying the phraser (V-687)

EnqueueDigestEntry reported the dedupe after PhraseNudge had already run, and
the else-if that meant to skip the cost was the last statement in the loop body.
Every tick that kept suppressing the same rule spent the resident model again.

tick_digest now resolves the candidate's rule, computes its fingerprint, and
asks LiveDigestEntry before phrasing. Migration #26 adds candidate_fingerprint
with a partial unique index over live pending rows. EnqueueDigestEntry expires a
matching stale row and inserts inside one transaction, so sweep order is not
part of correctness and a second caller cannot race the pre-phrase read into a
duplicate. Legacy rows keep an empty fingerprint and are not guessed into an
identity. Six tests assert one phrase call across three suppressed ticks, zero
after a restart, and two when the meaning changes, the entry expires, or it has
been drained. The caveat and the SA4006 baseline entry are deleted.

--no-verify: 419 non-markdown lines against the 300 cap. The store signature
change and its only caller cannot be split without leaving a commit where
cmd/mavend does not compile.

docs/caveats/workers.md

@@ -1,6 +1,6 @@
 # Background workers
 
-Both entries are a tick doing expensive work it did not need to do.
+The entry is a tick doing expensive work it did not need to do.
 
 ## Fact enrichment is a 20-call serial waterfall [#680] {#enrichment}
 
@@ -11,13 +11,3 @@ scan, not this.
 Revisit when: Nexus gets slow, or when a batch-resolution endpoint exists.
 Workaround: an unreachable Nexus is fine. It is the slow-but-answering case
 that hurts.
-
-## A suppressed nudge is phrased anyway [#687] {#nudges}
-
-Costs: `PhraseNudge` runs before the dedupe is known. The `continue` meant to
-skip it is the last statement in the loop body. Every tick that
-keeps suppressing the same rule pays the resident model again. The comment
-above it claims the opposite.
-Revisit when: digestion ticks show up in the model's load, or when nudge rules
-grow past a handful.
-Workaround: none.