docs(roadmap): record the changes waiting on an external trigger

[LINDECKER-Charles]

Adds docs/roadmap.md and links it from both indexes.

Two migrations are already decided but blocked on something outside this repo. Until now they existed only as a comment in ci.yml and a closed PR thread, which is enough to record that something was deferred but not enough to act on it later — a comment carries neither the trigger, nor the exact edits, nor the reason the obvious-looking version was the wrong one.

Entry	Trigger
Add Node 26 to the CI matrix	a date — 2026-10-28, when 26 reaches Active LTS
Port to TypeScript 7	an upstream release — when @typescript-eslint/* admits TS ≥ 7.1 (#10940)

The two are deliberately filed under different headings, because a calendar alone would not cover the second: TS 7 has no date. What it has is a one-command test, which the entry records so the check is not re-invented:

npm view @typescript-eslint/eslint-plugin peerDependencies.typescript

Each entry carries the trigger, the exact edits, the verification to run before merging, and — the part most likely to be lost — what must not change. For Node 26 that is the floor: engines.node, @types/node and the tsup target all track the oldest supported line, never the newest. Conflating those two is precisely what produced a @types/node PR proposing types for a runtime that was not even LTS yet, and the roadmap now states the rule instead of leaving it to be rediscovered.

The TS 7 entry also preserves the measurement taken while investigating it, so the work does not get redone: the source is already TS 7 clean (tsc --noEmit on 7.0.2 → 0 errors, tsup build fine), and the only blocker is the plugin throwing Error: typescript-eslint does not support TS 7.0. at load — which kills lint and lint:security, the security gate. That is why forcing peers is not an option.

Scope: documentation only. No code, config or dependency changes.

• linked from the README.md documentation table and the docs/README.md contributor index
• added to the reading-order mermaid diagram in docs/README.md, so every document in that index stays represented
• all five relative links out of the new file verified to resolve (scope.md, releases/, ../.github/workflows/ci.yml, ../.github/dependabot.yml, ../package.json)