Amends ADR-0075: which callbacks receive opts

[johnnyt]

Adds a second Amendment to ADR-0075, at proposed. It amends decision 8 point b in part, and records which callbacks a {module, opts} registration's options reach in the code statifier 2.10.0 ships.

What changes

• docs/adr/0075-basichttp-event-io-processor.md: one Amendment appended at the end of the file, in the shape of the Amendment above it (### Amendment 2026-09-30: ... with its own Status line, proposed (2026-09-30)). No line is removed.

The Amendment

Decision 8 point b says the options reach "the plan context a processor's callbacks receive, under an added :opts key". Statifier.Send.Processor has three callbacks that take its ctx(): deliver/3, cancel/2 and perform/2. The shipped code gives :opts to the first two only. The Amendment states that as the decision:

• the options reach deliver/3's and cancel/2's context under :opts;
• perform/2 receives the session's plan context without :opts, because a {:handler, module, payload} instruction does not name the registration it came from;
• a processor that needs its configuration at perform time carries it in the payload deliver/3 plans, as Statifier.Send.BasicHTTP does with its transport.

It decides nothing beyond which callbacks receive :opts.

Direction check

Every claim in the Amendment was checked at c8894aea, the v2.10.0 tag and main:

• Statifier.Session.Effects's private processor_for/2 puts the options under :opts only in the context deliver/3 and cancel/2 receive.
• Statifier.Session's private perform_instruction/3 calls module.perform(payload, plan_context(state)), and plan_context/1 has no :opts key.
• The Statifier.Send.Processor moduledoc, section "Registration options", names deliver/3 and cancel/2.
• Statifier.Send.BasicHTTP's private post/2 puts the transport into the {:post, ...} payload that perform/2 reads.

The Amendment cites each by function anchor, not by line. It removes no line (git diff origin/main --numstat shows 32 added and 0 removed). Its Status line says what it amends and what stays unchanged. No ruling is cited: the Amendment records what the shipped code does.

Gate

Docs only: no Elixir code changes. mix adr.check and mix quality --profile merge ran green on this tree. The ADR judge skipped: no file in this diff is in one of its judged scopes.