fix(simulation): address PR #171 review — all findings processed (T-987)

Every review finding fixed (no non-blocking parking lot):
- shell.rs: debug_assert the D-110 floor-range i8 invariant (H1); clarify the
  ground_offset fallback comment (T1) and the roof_z None-case (T3); clarify the
  sub-chunk clip test comment (T4); add a test for the elevated/no-ground-floor
  fallback (H2).
- scale.rs: add CHUNKS_PER_BLOCK (= BLOCK_M/CHUNK_M) + compile-time asserts; used
  by shell.rs fill_chunk's sub_chunk bounds assert (T5).
- gen_queue.rs: document why FillChunk's Vec<BuildingPropertyTag> needs no Box
  (large_enum_variant non-issue) (T2).

clippy --all-targets -D warnings clean; 1572 lib tests pass (+1).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/src/atlas/gen_queue.rs

@@ -133,6 +133,9 @@ pub enum GenWorkItem {
         sub_chunk: (u8, u8),
         /// Covering block's building tags, pre-resolved from the cached
         /// `QuarterWorldState`. Empty for an open/un-built block (→ empty shell).
+        /// A `Vec` is three words on the stack regardless of element size (the tags
+        /// live behind the pointer), so — unlike `AnalyzeBody`'s boxed `BodyParams` —
+        /// this variant needs no `Box` to stay clippy `large_enum_variant`-clean.
         block_tags: Vec<BuildingPropertyTag>,
     },
 }