docs: start doc/ — QC checks and the module map (WIP)

[ZheningHuang]

Draft — actively being written. Opening it early so the structure is visible and reviewable in pieces.

Somewhere to write down what the pipeline is made of and what each QC gate actually promises, rather than leaving it spread across module docstrings.

file	state
doc/modules.md	the eight modules the system is built from
doc/QC/collision_check.md	entities, rules, the deterministic function, what a failure hands the repair step
doc/QC/door_open.md	a one-line note — not yet a check
doc/Tools/*.md	six empty placeholders, one per agent tool — structure only

collision_check.md

The most complete one. It records the parts that are decisions rather than implementation details:

• which findings fail the gate and which don't — floating and outside_room are reported but do not fail, because a floating object means a missing support and outside_room means the scan's wall loop is wrong. Neither is repairable by the collision resolver, so failing on them would block every publish on an upstream problem.
• why contact and distance come from different geometry — contact from the raw triangles (the only model that lets a chair tuck under a table), distance from a convex decomposition (the raw meshes are not watertight, so a penetration depth taken from them is meaningless). The convex model is never allowed to decide whether two things touch.
• what a failure hands the fixer — the exact metric vector, not a description.

Lowercase doc/

Deliberate. This filesystem is case-insensitive (core.ignorecase=true), so a stray Doc/ looks identical locally but is a separate path on Linux CI. Nothing is tracked upstream yet, so fixing the case now costs nothing; after two directories exist it is a real cleanup.

Not here

doc/QC/support_check.md — it belongs with the code it describes, in #37.

Known gaps

• collision_check.md still says wall fixtures are "fixed to the wall". In the current code they are in no collision body at all — Panda-1 has five that are simply absent from the model. Needs correcting.
• The Tools/ files are empty.