docs(adr): record that FastMCP 4 fired ADR 0001's revisit trigger

[lesnik512]

ADR 0001 chose FastMCP.add_provider() for teardown and set a revisit trigger: FastMCP grows a
first-class shutdown hook, either an on_shutdown API or a documented public way to compose a
lifespan post-construction.

Checked against fastmcp 4.0.3. The trigger fired on its second clause, but the replacement it
pointed at is worse than what we have, so this records the evaluation and narrows the trigger rather
than changing the decision.

• on_shutdown and on_startup are still absent, so the first clause did not fire.
• add_extension(ServerExtension) exists, and ServerExtension.lifespan() is documented as
  "A context manager entered with the server's lifespan, exited on shutdown", to "start and stop
  resources an extension owns". That satisfies the second clause literally.
• But ServerExtension.identifier is a required reverse-DNS string, validated at registration, and
  documented as "advertised under ServerCapabilities.extensions". Registering one purely for a
  shutdown callback would announce a protocol capability the bootstrapper does not implement. The
  Provider route's thinness is invisible outside the process; this would not be.
• The fastmcp extra declares an unbounded "fastmcp", so ServerExtension may not exist at
  runtime. Adopting it would force a fastmcp>=4 floor on an optional extra.

The trigger now names a hook that is neither client-visible nor tied to another abstraction's
semantics, and says explicitly that add_extension does not qualify, so this does not get
re-litigated on every version bump.

Docs only, no code change.

[lesnik512]

Closing unmerged; the ADR amendment is not being pursued for now.