feat(simulation): interstitial fill — ground-tile character between building footprints (T-1098)

The BSP leaves that lose the D-233 coverage roll in
subdivide_block_footprints were computed and discarded; they are now
surfaced as the interstitial rect set (BlockSubdivision), making the
ground-plane classification exhaustive by construction: footprint /
interstitial / street-margin-or-reserved. FillChunk carries the leaves
plus a minimal BlockFillContext (interstitial_character + setback_tier,
re-derived at block level via the existing pure fn); FilledChunk gains
a sparse interstitial map whose absence contract is stated on the
struct (missing key = footprint/street/reserved, never unknown). The
pure resolution maps OperationsSurface (D-233) first, else setback_tier
onto five of D-235's seven interstitial values — dock_slip/market_pad
have no specified trigger in the record and point at T-1209 rather
than an invented mapping. Design brief with the geometry model at
docs/architecture/interstitial-fill-t1098.md (lead-approved
checkpoint). Bonus fix: a degenerate setback shrink previously vanished
from BOTH lists silently; it now falls through to interstitial. 14 new
tests; full cargo test green incl. all golden harnesses; purity per
T-987 (plan-time compute, pre-resolved work items, no cache reads).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

server/src/atlas/layer_proxy.rs

@@ -5185,6 +5185,7 @@ mod tests {
                     ..Default::default()
                 },
                 block_tags: Default::default(),
+                block_interstitial: Default::default(),
             },
         );
 
@@ -5252,6 +5253,7 @@ mod tests {
                     ..Default::default()
                 },
                 block_tags: Default::default(),
+                block_interstitial: Default::default(),
             },
         );
 
@@ -5320,6 +5322,7 @@ mod tests {
                     ..Default::default()
                 },
                 block_tags: Default::default(),
+                block_interstitial: Default::default(),
             },
         );
 
@@ -5386,6 +5389,7 @@ mod tests {
                     ..Default::default()
                 },
                 block_tags: Default::default(),
+                block_interstitial: Default::default(),
             },
         );
 
@@ -5627,6 +5631,7 @@ mod tests {
                     ..Default::default()
                 },
                 block_tags: Default::default(),
+                block_interstitial: Default::default(),
             },
         );