feat(street-level): provider contract with Mapillary as the first provider

Street Level is now a provider-neutral layer modelled on the iD editor's
photo overlay. The core owns what every imagery provider shares: the
enable state and a switch per provider, the imagery filter (360°/flat and
captured-since in relative days), one click handler routed by pick prefix,
one viewer host that mounts whichever provider's viewer adapter owns the
open image, the position marker, camera follow, one on-globe credit per
active provider, and share-link parameters (one bit per provider plus the
filter). Providers implement the contract in registry.js: coverage,
sequences, nearest-image lookup and a viewer adapter emitting a common
pose. Mapillary is the only registered provider; Google Street View,
KartaView and Panoramax slot in beside it.

Panel: a PROVIDERS chip row (one chip per registered provider, keyless
chips read as errors), relative-day SINCE options, a provider-neutral
caption whose link label follows the provider, and no vendor mark in the
header. The panel inner is a rail scroller like CCTV's.

Tests cover the registry, params codec, filter, pick router, UI-state
composition, the provider definition and coverage-tile decoding; the QA
harness switches the provider off and on and checks the filter.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

src/layers/streetLevel/policy.js

@@ -0,0 +1,26 @@
+/** Identity and shared tuning for the provider-neutral Street Level layer. */
+export const STREET_LEVEL_LAYER_ID = 'street-level';
+
+/** Pick id of the viewer position marker; the core owns it, not a provider. */
+export const POSITION_PICK_ID = 'sl:pos';
+
+/** Colours every provider shares: the selection highlight and the marker. */
+export const COLORS = Object.freeze({
+  selected: '#00d4ff',
+  position: '#ffb300',
+});
+
+/** Panorama modes the imagery filter understands. */
+export const PANO_MODES = Object.freeze(['all', 'pano', 'flat']);
+
+/** Longest "captured since" window the filter accepts, in days (~100 years). */
+export const MAX_SINCE_DAYS = 36_500;
+
+/** Filter every provider starts with: all imagery, any date. */
+export const FILTER_DEFAULT = Object.freeze({ pano: 'all', sinceDays: 0 });
+
+/** Nearest-image search when the user asks to look at a place. */
+export const NEAREST_RADIUS_M = 50;
+
+/** Eye height above the sampled ground when the globe camera follows the viewer. */
+export const FOLLOW_EYE_HEIGHT_M = 2.4;