fix(cockpit): a coarse read never takes a river bridge back to the sea

Before the first full-detail water read lands (it can take many seconds
from cold tiles), a refresh's loaded-tile read with one low probe still
dropped a river height to the geoid. Before that landing, loaded-tile reads
may now find raised water but only a full-detail read lowers it.

CURRENT-STATE also says what the earlier fixes changed: C stays the CCTV
shortcut when the bridge cannot be entered, voice entry is refused on a
bridge, and the ridden ship's trail is hidden with its sprite.

docs/CURRENT-STATE.md

@@ -1530,16 +1530,20 @@ With no aircraft tracked, Cockpit can ride the selected AIS vessel instead.
 Entry needs only the layer of the vessel it rides, Live AIS Vessels, and a
 selected vessel; Contacts and every other layer may be off, before or during
 the bridge (`bridgeEntryAllowed` in `src/contextModePolicy.js`). A Context mode
-change still in flight blocks entry until it settles. `C` enters it, and the
+change still in flight blocks entry until it settles. `C` enters it (when
+the bridge cannot be entered, `C` stays the CCTV shortcut), and the
 entry button reads BRIDGE with a ship icon both in Contacts (`#cockpit-entry`)
 and under the selected vessel's Intel HUD readout in the bottom-right corner
 (`#cockpit-entry-readout`, shown with the HUD on, in its Tactical and Cyber
 layouts). Turning Live AIS Vessels off ends the bridge: the layer then has no
 subject (`getCockpitSubject` returns null). The bridge picture uses AIS alone,
 so no other feed being off makes it partial.
-Voice (`control_cockpit`) and the selection entry transaction stay aircraft-only.
+Voice (`control_cockpit`) and the selection entry transaction stay aircraft-only;
+voice `enter` on a bridge is refused before it touches tracking, with the
+reason to exit the bridge first.
 The vessel layer supplies the subject (`getCockpitSubject`), hides that ship's
-sprite, card and detection bracket while it is held (`setCockpitSubject`), and
+sprite, card, detection bracket and trail while it is held
+(`setCockpitSubject`), and
 ignores scene clicks so open water cannot deselect it. The eye sits on the AIS
 position at a representative bridge height for the AIS type family
 (`src/layers/vessels/cockpitSubject.js`: cargo and tanker 30 m, passenger 25 m,
@@ -1569,8 +1573,9 @@ eight probes 30 m out, the lowest probe taken as the water when it is 2 m to
 the full-detail read replaces it; it repeats every 5 s. Only one full-detail
 read runs at a time: while one is still running, a refresh reads the loaded
 tiles instead, and the slow read applies whenever it lands. Once a
-full-detail read has answered, loaded-tile reads no longer change the height,
+full-detail read has answered, loaded-tile reads no longer change the height;
-and a read that sampled nothing keeps the height it had. A failed read keeps
+before then they may find raised water but never take it back to the sea.
+A read that sampled nothing keeps the height it had. A failed read keeps
 the sea surface.
 
 The HUD keeps its frame, vision styles, briefing, weather and Display/Radio

src/ui/cockpitBridge.test.mjs

@@ -1350,6 +1350,30 @@ test('a river height stands through reads that sample nothing or one coarse prob
   );
 });
 
+test('before any full read lands, a coarse loaded read keeps the river it found', async (t) => {
+  const clock = manualClock(t, START_MS);
+  const sea = await bridgeAt(t, clock);
+  frames(sea.cockpit, clock, sea.shots, 60);
+  const seaEyeM = eyeHeightM(sea.shots.at(-1));
+  sea.cockpit.exit();
+
+  clock.set(START_MS);
+  const record = { ...RECORD, lastPositionEpoch: START_MS / 1000 };
+  const ship = fixture(t, { record });
+  // The full-detail read is cold and never lands in this test.
+  ship.viewer.scene.sampleHeightMostDetailed = () => new Promise(() => {});
+  ship.viewer.scene.sampleHeight = () => 27;
+  ship.cockpit.enter();
+  await settle();
+  ship.cockpit.update();
+  frames(ship.cockpit, clock, ship.shots, 60);
+  near(eyeHeightM(ship.shots.at(-1)) - seaEyeM, 27, 'on the river', 0.5);
+  // A refresh meets one low probe on coarse tiles.
+  ship.viewer.scene.sampleHeight = () => 0.5;
+  frames(ship.cockpit, clock, ship.shots, 240);
+  near(eyeHeightM(ship.shots.at(-1)) - seaEyeM, 27, 'still on the river', 0.5);
+});
+
 test('a client clock minutes fast does not age the bridge into a stale feed', async (t) => {
   // The feed's newest fix is this ship's; the client clock runs 200 s
   // ahead of the feed and received the refresh just now.

src/ui/cockpitCamera.js

@@ -110,9 +110,10 @@ export function update() {
  * every frame until it settles): while one is still running, a refresh
  * reads the loaded tiles instead, and the slow read applies whenever it
  * lands. Once a full-detail read has answered, only full-detail reads
- * change the height: a loaded-tile read stands in only until the first
+ * change the height; before then a loaded-tile read may find raised water
- * lands, so one coarse probe cannot drop a river bridge to the sea, and a
+ * but never takes a raised height back to the sea. One coarse probe cannot
- * read that sampled nothing keeps the height it had.
+ * drop a river bridge to the sea, and a read that sampled nothing keeps
+ * the height it had.
  * @param {{latitude: number, longitude: number}} [at] Where to read the
  *   water: the dead-reckoned position the camera rides.
  * @returns {number} Ellipsoidal metres.
@@ -145,7 +146,12 @@ function vesselSurfaceM(info, nowMs, at = info) {
       read.sampledMs = nowMs;
       if (read.inFlight && !read.landed) {
         const loadedM = loadedWaterlineM(scene, probes, seaM);
-        if (Number.isFinite(loadedM)) read.heightM = loadedM;
+        // Taking a river back to the sea is the full-detail read's to say.
+        const lowers =
+          Number.isFinite(read.heightM) &&
+          read.heightM > seaM &&
+          !(loadedM > seaM);
+        if (Number.isFinite(loadedM) && !lowers) read.heightM = loadedM;
       }
     }
     if (!read.inFlight) readWaterline.call(this, read, scene, probes, seaM);