P2 pre-merge fixes: io_captured column, reconcile copy, harden security test

Fix 1: Add io_captured TINYINT(1) NOT NULL DEFAULT 0 to prv_call_meta
CREATE (baked into initial schema — table is new in v0.3.0, no separate
migration needed). PRV_Capture_Writer::write_io() sets io_captured=1 via
set_io_captured() on success; error rows and legacy rows keep 0.
Drawer state now driven by io_captured instead of config_version heuristic:
  0         → Legacy/not-captured
  1 + io    → Normal
  1 + no io → Pruned
ARCHITECTURE.md schema section updated.

Fix 2: Soften reconciliation copy in PRV_Costs_Renderer (~line 75):
'matches the Dashboard MTD total' → 'approximately reconciles to the
Dashboard MTD total' — error calls write prv_call_meta (cost 0) but not
prv_ai_visibility, so exact equality is not guaranteed.

Fix 3: Harden security test (test-capture-writer Test 4): inject sentinel
sk-or-TEST-LEAK-CANARY as PRV_OPENROUTER_API_KEY constant; queue a mocked
401 wp_remote_post response; drive PRV_Probe_Runner → PRV_Probe_Run_Executor
→ probe_one() 401 error path end-to-end; assert the sentinel, Bearer, and
Authorization appear in NO stored row (prv_call_meta or prv_call_io).
Original direct-writer security test retained as Test 3.

includes/core/class-prv-costs-renderer.php

@@ -83,7 +83,7 @@ class PRV_Costs_Renderer {
 			echo esc_html(
 				sprintf(
 					/* translators: 1: cost, 2: call count */
-					__( '$%1$s recorded across %2$d calls this month — matches the Dashboard MTD total.', 'pr-vision' ),
+					__( '$%1$s recorded across %2$d calls this month — approximately reconciles to the Dashboard MTD total.', 'pr-vision' ),
 					number_format( $mtd_cost, 4 ),
 					$total_calls
 				)