fix(simulation): PR #216 review fixes — footprint-wins enforcement, discriminating tests

Finding 2 became a real code fix: interstitial_fill_into now enforces
the footprint-wins conflict rule (column_has_voxel range probe) — the
FilledChunk absence contract was previously a documented promise the
code didn't keep against conflicting inputs; pinned by a fully-
overlapping-leaf test asserting per-tile resolution. The tautological
overlap test replaced with a real rects_overlap() geometric helper
(itself sanity-tested) applied pairwise. The degenerate-setback fix is
now a standalone pure fn shrink_lot_or_interstitial with four boundary
tests — honestly documented as unreachable from live traffic today
(every min_lot exceeds every setback), a robustness guard for future
recalibration. Both sub-chunk clip tests now reconstruct the full
32-tile union across the seam (disjoint + complete), including the
pre-existing footprint clip test (leave-cleaner). Brief's ChunkLayout
claim tightened to the verified no-production-consumer statement.

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

docs/architecture/interstitial-fill-t1098.md

@@ -77,8 +77,12 @@ by *construction*, with no residual "what's left over" category to guess at:
    (`skeleton_gen.rs:872-887`, `// Perimeter street margin`) — it is claimed by
    the future street-network layer (D-234), not by this ticket. `ChunkLayout`
    (`spacing`/`offset`/`rotation_steps`, `generator.rs:103`) is the *only* other
-   street-adjacent data on `BlockSkeleton` today, and it is an unconsumed stub
-   (grep confirms no reader) reserved for that same future street-network
+   street-adjacent data on `BlockSkeleton` today; grep confirms it has **no
+   production consumer** (its only reads are `#[cfg(test)]` assertions in
+   `skeleton_gen.rs` — `skeleton_has_streets_and_local_lattice` — checking that
+   the plan-time writer set it; the writer itself, `generate_quarter_skeleton`'s
+   layout-mode application, populates it but nothing downstream of plan time
+   consumes it yet). It is reserved for that same future street-network
    step — not something this ticket needs to or should resolve.
 
 Every block-local tile is covered by exactly one of these three at generation