Ship the SR-003 schema bump: jmanifest_version 2

Moves the exchange envelope to version 2 in lockstep with the truth file's
schema_version, per SR-003's requirement that breaking changes be batched and
ship together rather than piecemeal. The plugin had already moved to
schema_version 2; the server declaring 1 while accepting the new fields
defeated the point of having a version at all.

Flag day, not dual-accept (JR-003): version 1 is now rejected outright. All
three components are pre-release, and a v1 read path would be the one nobody
exercises, so it is the one that would rot while being dragged through every
later change to the reader. A pipeline still emitting v1 is incompatible until
updated — stated plainly rather than papered over with a shim nobody tests.

scenes become objects carrying belief and route (extraction AR-017) instead of
float pairs. Belief is bounded to [0, 1] rather than merely stored: §5a's
Threat 1 argument rests on every accepted value being bounded, and an unbounded
float is a 64-bit channel however harmless it looks. route is a closed enum, so
an invented value cannot be stored.

UR-018 is the requirement with the trap in it, and the reason content_id.rs is
untouched by this commit: belief is a producer-side estimate that may
legitimately differ between pipeline versions for identical timings, so
including it in the canonical form would give two servers different ids for the
same content — the exact failure mode §9a quantises centiseconds to avoid,
reintroduced one field along. It replicates as an attribute, exactly as
audio_signature does. The golden vector still passes unchanged, which is the
evidence rather than the claim.

191 tests. UR-015..018 move from Planned to Done; coverage 24/32 (75%).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

TRACES: UR-014, UR-015, UR-016, UR-017, UR-018 | SR-003

src/db/schema.sql

@@ -70,7 +70,13 @@ CREATE TABLE IF NOT EXISTS scenes (
     manifest_id     TEXT NOT NULL REFERENCES manifests(id) ON DELETE CASCADE,
     tmdb_person_id  INTEGER NOT NULL,
     start_cs        INTEGER NOT NULL,
-    end_cs          INTEGER NOT NULL
+    end_cs          INTEGER NOT NULL,
+    -- Per-window provenance (SR-003, extraction AR-017). Deliberately NOT part
+    -- of `content_id`: belief is a producer-side estimate that may differ
+    -- between pipeline versions for identical timings, so hashing it would give
+    -- two servers different ids for the same content (§9a).
+    belief          REAL,
+    route           TEXT            -- live | deferred | pooled
 );
 
 CREATE TABLE IF NOT EXISTS reports (