refactor(tools): group the sixty scripts by what they talk to

One directory with sixty scripts stopped being navigable, and the grouping is also the sandbox boundary:

  Tools/unreal/    commandlets and the two map builders, with ue_common.py and ue_placement.py as the
                   shared imports (they need the engine)
  Tools/blender/   builders, exporters, the FBX preset, the MPFB character pipeline, blender_screenshots
                   (they need the Blender Flatpak)
  Tools/review/    the visual loop: render_screenshots.sh, judge_screenshot.py, hero_compare.py,
                   build_comparison_md.py, e2e_asset_check.py, hero_iterate.sh, visual_loop.py,
                   visual_targets.json
  Tools/gen/       image and texture generation: the ComfyUI driver, deterministic texture writers,
                   reference fetching
  Tools/input/     OS-level input injection
  root             _common.py, check-lfs.sh, addon_compat_probe.py, sibling_asset_audit.py, tests/

Every moved script now resolves the repository root as ../.. and puts the tools root plus its siblings on
sys.path, so 'import ue_common' still works from unreal/. The test suite bootstrap does the same for the five
folders. render_screenshots.sh takes the folder as a third argument to run_cmdlet, and the e2e driver,
hero_iterate.sh and the comparison generator were repointed.

51 README rows now link to their new location, the README gains a Layout section naming each folder's
sandbox, and the architecture notes and Blender README follow the moves.

Verified: 108 tests pass, prettier is clean, judge_screenshot.py and hero_compare.py run from their new
homes, and the e2e run was aborted mid-way (it had been rebuilding an empty DemoMap for 15 minutes) and is
being rerun against the new layout.

Tools/unreal/ue_placement.py

@@ -0,0 +1,65 @@
+#!/usr/bin/env python3
+"""Single source of truth for where staged actors belong in the DemoMap.
+
+This exists because the same value was written in two places and they drifted:
+`ue_hero_facade_import.py` kept the pre-`0.1.0` position `(-12024.5, -1450)` while
+`ue_build_demo_map.py` staged the hero at `(-13336, 1307)`. Running the importer on
+its own therefore moved the hero **13 m east and onto the north side of the
+boulevard** — and nothing complained, because a displaced building still renders.
+
+The values are Unreal centimetres, absolute world space. Every one of them is
+verified against `Geo/verification/2026-11-06-el-capitan-block.md` and probed by
+`Tools/review/visual_loop.py --verify` (via the demo screenshot expectations) and by the
+world-span probes recorded in `MEMORY.md`.
+
+Keep this file boring: constants and one validator, no `unreal` import, so it can
+be unit-tested on the host.
+"""
+
+from __future__ import annotations
+
+# The hero façade sits INSIDE the frontage gap between 6902 (ends at -17749) and
+# the El Capitan Theatre (starts at -12082), so its actor origin is the measured
+# building line. The mesh is authored with its front plane 7 cm ahead of the actor
+# origin, which is why the X here is the frontage centre and Y is the building line.
+HERO_ACTOR = "Hero_Facade"
+HERO_LOCATION_CM = (-13336.0, 1307.0, 0.0)
+
+# Tolerance for the span probe: the verified footprints are within ~1 m.
+SPAN_TOLERANCE_CM = 100.0
+
+# Expected world span per frontage actor (min X, max X), from the block
+# verification notes. Used to prove a staged building is on its own footprint
+# rather than merely present.
+FRONTAGE_SPANS_CM = {
+    "6922": (-24276.0, -17700.0),
+    "6902": (-17749.0, -14591.0),
+    "hero": (-14644.0, -12028.0),
+    "6838": (-12082.0, -8210.0),
+    "6822": (-7854.0, -7252.0),
+    "6806": (-6608.0, -3307.0),
+    "6800": (-3320.0, -1437.0),
+}
+
+
+def span_error(name: str, observed_min: float, observed_max: float):
+    """Return (expected_min, expected_max, ok) for an observed building span.
+
+    `name` is a key of `FRONTAGE_SPANS_CM`; `ok` is False when either edge is more
+    than `SPAN_TOLERANCE_CM` from the verified value.
+    """
+    if name not in FRONTAGE_SPANS_CM:
+        raise KeyError(f"unknown building {name!r}; known: {sorted(FRONTAGE_SPANS_CM)}")
+    exp_min, exp_max = FRONTAGE_SPANS_CM[name]
+    ok = (abs(observed_min - exp_min) <= SPAN_TOLERANCE_CM
+          and abs(observed_max - exp_max) <= SPAN_TOLERANCE_CM)
+    return exp_min, exp_max, ok
+
+
+def describe_span_failure(name: str, observed_min: float, observed_max: float) -> str:
+    """A one-line message for a misplaced building, or "" when it is correct."""
+    exp_min, exp_max, ok = span_error(name, observed_min, observed_max)
+    if ok:
+        return ""
+    return (f"{name}: span X {observed_min:.0f}..{observed_max:.0f} does not match the "
+            f"verified {exp_min:.0f}..{exp_max:.0f} (±{SPAN_TOLERANCE_CM:.0f} cm)")