v1.9.1 - a front door worth walking through

A documentation release. Nothing in the library's behaviour changed - the DLL
differs from 1.9.0 only in its version stamp and doc-comment metadata.

An outside assessment put it plainly: the hard parts of this documentation were
unusually deep and the easy parts were full of holes. The landing page read
like an API reference merged with an audit report, and the prominent example
produced six compiler errors as pasted. Both are fixed at the root.

The README is a landing page again - what the library is, the demo in
motion, a compatibility box, installation, a quickstart, and a map. The
reference moved next door and is better for it:

• docs/API.md -
  members, options, events, failure causes, channels, threading, ordering
• docs/RECIPES.md -
  HUDs and transparency, shaping, real files and web fonts, previewing without
  the game, performance, security defaults
• docs/TROUBLESHOOTING.md -
  by symptom, ending in what a useful bug report contains
• docs/INTERNALS.md -
  how it works and why it looks like this, plus the review history

The quickstart compiles as pasted, and stays that way. It is a complete
plugin - reference block, hotkey, failure handling, Dispose on shutdown -
and it is not trusted but extracted: tools/Check-Quickstart.ps1 compiles the
two marked README blocks verbatim against a real SPT installation, and the
release packaging refuses to package when they do not build.

IntelliSense. Anvil-WebOverlay.xml now ships next to the DLL, so
referencing the installed library gives the documentation comments in the
editor rather than on a website.

Every dated review report under docs/ now carries its own
historical-snapshot banner, because a reader deep-linked into one never passes
through the index that says so.

Extract over the game root, as always. Nothing a consumer gates on has moved:
1.8.8 remains the floor for click-through, 1.9.0 for VirtualHost.Access.