Add an app compatibility matrix: which insertion route, paste confirmation, caret and context each kind of app actually gives

## What is missing

Dictation, clipboard paste-back and AI suggestions all depend on how *other* apps publish text through Accessibility, take a synthetic ⌘V, and report a caret rectangle. What is known is scattered over seven documents, each measured separately and some of them contradicting each other:

- `Docs/insertion.md` and `Docs/input-paste-eligibility.md`: which apps refuse the Accessibility write.
- `Docs/context-accessibility.md`: which apps give a window title and a selection (a three-row table).
- `Docs/predict-ime.md`: which toolkits publish `AXTextInputMarkedRange`.
- `Docs/predict-reliability.md`: browser editors, apps built on a bundled browser engine, and cross-platform-toolkit IDEs that publish nothing.
- `Docs/predict-probe.md`: the surface capability table, still empty (#101).
- `Docs/input-synthetic-keystrokes.md`: what posted events contain, with no per-app results.

```
$ git grep -n -i "compatib" -- Docs README.md CONTRIBUTING.md
(no output)
```

There is no single place that says "in this kind of app, this is what works". Nor is there a way for a contributor to add a row by testing one app they already use.

## Proposal

A new `Docs/compatibility.md`: one table per app class, one row per app version tested, with no marketing claims, filled only from runs of the existing tools (`uttrflow-dev insert --via …`, `uttrflow-dev probe surface`, `uttrflow-dev probe ime`) and a short manual check.

| Column | Values | How it is measured |
|---|---|---|
| App, version, macOS version | text | About window |
| Class | browser (engine) / bundled-browser-engine app / native AppKit / cross-platform toolkit / terminal / office / remote or VM / game | by inspection |
| Field tested | e.g. single-line, multi-line, rich editor, spreadsheet cell, secure | by inspection |
| Focused field published | yes / no / only after `AXManualAccessibility` | `uttrflow-dev probe surface` |
| Accessibility write | lands / refused / reports success, changes nothing / lands late | `uttrflow-dev insert --via accessibility` |
| Paste | lands / ignored / other shortcut fires / forwarded elsewhere | `uttrflow-dev insert --via paste` |
| Paste confirmation | landed / not reported / gave up | same run's printout |
| Full route result | once / twice / nowhere | `uttrflow-dev insert` |
| Caret rectangle | right / wrong place / none | `uttrflow-dev probe surface` plus a look |
| Value and selection readable | yes / value only / no | `uttrflow-dev probe surface` |
| Marked text (IME) published | yes / no | `uttrflow-dev probe ime` |
| Typed completion | correct / wrong characters / nothing | accept a suggestion |
| Notes | line breaks, undo, RTL, anything surprising | |

Each cell that is not a measurement stays blank. "Unknown" is a valid answer; a guess is not.

## Acceptance criteria

- `Docs/compatibility.md` exists with the columns above, a short "how to add a row" section, and at least the rows the maintainers can fill from the existing docs, each linked to the document it came from.
- The seven documents above link to it and stop keeping their own partial tables, or say which column they feed.
- `Docs/README.md` lists it, and the per-class testing issues filed alongside this one link to it.
- The docs audit in `make verify` passes.

Start with Docs/insertion.md, Docs/input-paste-eligibility.md, Docs/context-accessibility.md, Docs/predict-ime.md, Docs/predict-reliability.md, Docs/predict-probe.md, and Docs/input-synthetic-keystrokes.md. Run the existing uttrflow-dev insert and probe commands while checking Docs/README.md and the make verify audit. Done means Docs/compatibility.md has the specified columns, measured rows and contribution instructions, with the seven source documents and README linked or updated as described.

Written by the indexing model from the issue text.