feat(twap-oracle): implement RecordTick instruction

Add RecordTick — a permissionless instruction that reads the current tick
from a CurrentTickAccount and advances a PriceObservations ring buffer.

Authorization is implicit: both PDAs are verified against price_source_id,
so the tick can only have been written by whoever controls that price source.
A sampling guard silently no-ops if less than `window_duration /
OBSERVATIONS_CAPACITY`` ms have elapsed, allowing keepers to call blindly on
every block. Tick-delta truncation clamps the per-observation delta to
`MAX_TICK_DELTA (9 116)` before advancing tick_cumulative, with
last_recorded_tick tracking the untruncated position for the next delta.

Also switches ObservationEntry.tick_cumulative to use elapsed milliseconds
rather than seconds.

Closes #116

artifacts/twap_oracle-idl.json

@@ -95,6 +95,39 @@
         }
       ]
     },
+    {
+      "name": "record_tick",
+      "accounts": [
+        {
+          "name": "price_observations",
+          "writable": false,
+          "signer": false,
+          "init": false
+        },
+        {
+          "name": "current_tick_account",
+          "writable": false,
+          "signer": false,
+          "init": false
+        },
+        {
+          "name": "clock",
+          "writable": false,
+          "signer": false,
+          "init": false
+        }
+      ],
+      "args": [
+        {
+          "name": "price_source_id",
+          "type": "account_id"
+        },
+        {
+          "name": "window_duration",
+          "type": "u64"
+        }
+      ]
+    },
     {
       "name": "update_current_tick",
       "accounts": [