fix(regime): reseed stored history on a sensor change, and stop faking an observation

Two review findings on 46ace50.

[P1] Raising HY_OAS_WINDOW_DAYS to 700 only reached newly computed rows. A
routine run recomputes the latest trading date alone, and `rebuilding` was keyed
on "no v3 snapshot exists at all", which is false once the cutover has run --
so every row already written kept the credit gap the wider window exists to
close, indefinitely.

Adds SENSOR_REVISION: stamped into each snapshot, absent on pre-marker rows
(read as 1), and a stored revision below the current one triggers exactly one
reseed. Deliberately not METHODOLOGY, which would partition the history API and
discard the cached event study -- neither warranted, since the study recomputes
its Warning series from source rather than reading snapshots and so cannot be
staled by a reseed.

The reseed is bounded by REBUILD_LOOKBACK_DAYS in calendar days rather than a
session count, because the binding constraint is the OAS fetch: each replayed
row needs W3's 20-business-day lookback inside HY_OAS_WINDOW_DAYS. Replaying by
session count would have left the oldest stored rows unrepaired -- the exact
rows the fix targets. At 672 days the replay covers ~464 sessions, W3's oldest
requirement lands on the first fetched OAS day, and the ~400-session series the
cutover wrote is fully covered. A test asserts that relationship so the two
constants cannot drift back into recreating the gap.

[P2] With nothing ever collected, current_observation returned available=true
and the default placeholders -- "unknown" for every hyperscaler, "mixed" for the
reaction -- so the card announced a reading that never happened. Those are the
absence of an observation, not an observation of absence. Gated on `observed`
(non-null fetched_at, the one field every path writing real content stamps),
which blanks the content and drives a proper empty state naming where an admin
collects one. This was a regression from 46ace50; fundamental_overlay never had
it, since no observation means no effective date means pending.

Also renames the leftover v2 identifiers in the touched paths
(rewrite_existing_v2, latest_v2).

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

frontend/src/lib/types.ts

@@ -500,6 +500,9 @@ export interface RegimeFundamentalOverlay {
   reasoning: string | null;
   source: string | null;
   fetched_at: string | null;
+  /** Whether anything was actually collected. Live reading only; the snapshot's
+   *  point-in-time overlay omits it. */
+  observed?: boolean;
   observed_in_snapshot?: boolean;
 }