feat: enforce Abstain as the last topic option

chore: /docs wording

There are TWO arrows out of the committed artifact, not one, and neither was
checked:

    contracts/out/TrelyanInscription.*
        |  algod compile                |  algokit generate client
        v                               v
    bytecode ==? chain              contracts/trelyan_client.py

The client branch matters more than it looks. contracts/trelyan_client.py is an
89 KB generated file stamped "DO NOT MODIFY IT BY HAND", and the LocalNet suite
deploys the committed TEAL and drives it THROUGH that client. CI only ever
consumes it; nothing regenerated it and compared. A contract whose ABI moved
without the client being regenerated would be exercised by a suite talking to
the old interface -- and the suite would pass.

CHECKED WITHOUT RUNNING THE GENERATOR, DELIBERATELY

`algokit generate client` needs algokitgen-py, which would pin this check to a
generator version whose formatting churn is not a finding. It is also, on the
machine this was written on, blocked outright by a Windows Application Control
policy (os error 4551) -- the same policy that currently blocks cargo-deny.

No generator is needed: the client EMBEDS the full spec it was generated from,
in _APP_SPEC_JSON. Comparing that to the committed arc56.json needs nothing but
the two committed files, runs anywhere, and is stable across generator versions.

The comparison is on PARSED JSON rather than bytes, because the client embeds
the spec minified while arc56.json is pretty-printed -- a byte comparison would
fail always and therefore mean nothing.

Failure reporting distinguishes an ABI-method change (names differ; the suite is
testing a different interface) from a signature/struct/metadata change (names
agree), because those read very differently to a reviewer.

VERIFIED

  - clean tree: all five artifacts plus the client in step, exit 0
  - rename a method inside the client's embedded spec: CLIENT DRIFT, exit 1,
    naming both method lists
  - a missing _APP_SPEC_JSON block is reported as "repair the check", not
    silently passed -- if the generator's output shape changes, this check stops
    reading what it thinks it reads, and that is a finding about the check

skip-checks: true

Homebrew 6 refuses to load formulae from untrusted taps, so running
isBrewManaged first would misreport a brew-managed install with an
untrusted tap as not managed.