Rename scrap-drop entity to "debris" in code and config

Implements the requirements rename in the codebase. The salvageable object
dropped by destroyed ships and defence stations is now the "debris" entity;
the scrap resource it yields (cargo, delivery, smelting, threat,
scrap_drop_formula, scrap_per_threat) is unchanged.

Entity renames: ScrapDataComponent->DebrisComponent, ScrapSystem->DebrisSystem,
ScrapInfo->DebrisInfo, ScrapSelectionChangedEvent->DebrisSelectionChangedEvent
(member scrap->debris), spawnScrap->spawnDebris, getScraps->getDebrisSystem,
getAllScrapInfo->getAllDebrisInfo, scrapAtWorldPos/scrapInBox->debris*,
SalvageScrapBehavior::scrapTarget->debrisTarget, PendingCollection::scrap->debris,
ScrapTest.cpp->DebrisTest.cpp. Config key scrap_despawn_seconds->
debris_despawn_seconds (WorldConfig.scrapDespawnSeconds->debrisDespawnSeconds).
The salvage/deliver behavior classes keep their names (they act on the scrap
resource).

Also implements the new REQ-UI-DEBRIS-PANEL behavior in SelectedBuildingPanel:
a single selected piece of debris shows a "Debris" heading plus a "Scrap" stat
row; a multi/mixed field selection appends "Debris x N" and "Scrap x N" lines.

Updates the four renamed REQ-ID references in comments/docs, plus architecture.md
and derived.md. All 449 test cases pass; app, tests, and balancing targets build.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y7N59FsLA5e2kuVdqe4Uhc

bin/app/data/config/stations.toml

@@ -4,7 +4,7 @@
 # a fresh player defence station holds one early parity wave unaided; the
 # enemy station at level 0 matches the player station exactly and scales
 # with the push level x. Station scrap drops stay authored (pushing rewards
-# are tuned independently of ship production costs, REQ-RES-SCRAP-DROP).
+# are tuned independently of ship production costs, REQ-RES-DEBRIS-DROP).
 
 [hq]
 surface_mask = [