Remove obsolete collision remapping code and tests

- Deleted collision remapping tests (obsolete with hash IDs bd-8e05)
- Simplified collision.go from 704 to 138 lines
- Removed RemapCollisions, ScoreCollisions, and reference update code
- Removed issue_counters table dependencies (bd-807b)
- Added COLLISION_MATH.md documentation
- Fixed RenameCounterPrefix and ResetCounter to be no-ops
- Closed bd-a58f, bd-3d65, bd-807b

Hash-based IDs make collision remapping unnecessary since collisions
are extremely rare (same ID = same content).

Amp-Thread-ID: https://ampcode.com/threads/T-cbb0f111-6a95-4598-b03e-c137112f9875
Co-authored-by: Amp <amp@ampcode.com>

internal/storage/sqlite/compact_test.go

@@ -9,6 +9,8 @@ import (
 	"github.com/steveyegge/beads/internal/types"
 )
 
+const testIssueBD1 = "bd-1"
+
 func TestGetTier1Candidates(t *testing.T) {
 	store, cleanup := setupTestDB(t)
 	defer cleanup()