v0.7.0 — solid buildings, a working cache, and one query you can follow

You can walk into the buildings, the clock sweep finally evicts, and one query
can be followed across the whole city.

Fixed

• Buildings are solid. Nine of the city's landmarks could be walked straight
  through. The collision builder classifies an oversized mesh as needing a split
  and then recurses into its children — and a district that merges its whole
  structure into one geometry has no children, so the loop was empty and nothing
  was ever added. Thirty-three oversized meshes were being silently discarded,
  including the standby, the recovery ground, the backup vault, the WAL vault
  and the disk array. A collision query along the line the walker took agreed
  with him: there was no building there. Merged meshes are now split by their
  own triangles. Collider count went from 765 to 989.
• Slopes work. There were no surface normals anywhere in the collision code
  and no maximum-slope rule, only a fixed tolerance — which made the climb limit
  a function of speed: about 52 degrees at a run and 74 at a walk, and nothing
  ever slid.
• The buffer pool now behaves like a cache. The five demo relations totalled
  98 MiB while the shared_buffers slider starts at 128 MiB, so the smallest
  pool a user could pick was already bigger than the whole database. Every
  slider position gave an identical sample, the clock sweep never evicted
  anything, no backend ever wrote a victim page, and the no-bgwriter scenario
  produced nothing at all. The working set is now larger, and the access skew is
  tuned so the hot set still fits: 98.9% hit ratio with 7,602 evictions,
  which is what a real server looks like.
• The labels stopped covering the city. Roles were told to stop truncating,
  which was right on its own, but in aggregate every label then rendered name,
  role and readout at every zoom — fifteen at once obscured more than the side
  panels ever had. Detail follows attention now.
• Every mode has a visible way out. The observability page's only exit was a
  tooltip on the wordmark, which does not exist on touch. A test now enumerates
  the modes and asserts each has a reachable exit control, not merely a key
  binding.

Added

• Trace a query. Pick a statement and follow it from the client terminal
  through parse, plan, buffer reads, WAL and commit, narrated by the
  transaction's own state machine rather than a script. Three people asked for
  this independently, and a fourth said "even at 0.1x it is too quick — I should
  be able to fire individual transactions and watch them flow", which is why it
  has a step mode.
• The version and build hash are on screen, so a bug report can say which
  build it came from.

Changed

• The sample-versus-pool mistake is now unwritable. A reviewer found
  thirteen readouts whose label made a claim their computation did not satisfy —
  a pool figure computed from the visualisation sample — after four had already
  been fixed one at a time. The sample-scale counters now carry a nominal type
  and the compiler rejects the mistake, with contract tests that assert a
  readout named for the pool actually moves when the pool moves.