Adds ADR-0075: the Basic HTTP processor record

[johnnyt]

Summary

ADR-0075, at proposed, records the Basic HTTP Event I/O Processor
(SCXML appendix C.2) as a registered send type in this package, written
from the spike's findings on st-lhwd. ADR-0069 gains a dated foot Note
that names its loose "No conformance result moves" clause and points at
ADR-0075 as the answer to its reopen trigger. docs/adr/README.md
indexes the new record. No lib/ or test/ change, and no changelog
fragment (changelog.d/README.md excludes ADRs).

Refs: st-z0wg, st-swl8 (the ADR-0069 Note rides this PR and closes with it)

What ADR-0075 decides

1. Placement. statifier-ex owns Statifier.Send.BasicHTTP and its
   pure inbound decoder; statifier_router owns the durable-execution
   inbound front, which calls that decoder.
2. Type strings. The spec's URI and the short form basichttp, both
   registered; neither is a built-in spelling.
3. _ioprocessors. Both keys, one location (the base URL, /, the
   session id), written through a new optional ioprocessors_entry/2
   callback that receives the session id and the registration's options.
4. Outbound mapping (C.2.2). event to _scxmleventname, namelist
   and params to form parameters, <content> to the body (the name then
   in the query string), a missing target to error.communication
   planned in deliver/3, a failed delivery to error.communication
   through Statifier.Session.failed_send/3.
5. Inbound decoder (C.2.1). Statifier.Send.BasicHTTP.decode/1,
   pure, with the status rule a front applies (204, 405 with
   Allow: POST, 400, the front's 404).
6. Transport. A behaviour whose always-compiled default is OTP
   :httpc with TLS verification set; no new dependency; a host injects
   any other adapter.
7. Corpus. host.event_io_processors, a closed set whose one member
   is the Basic HTTP URI, allowed on W3C cases (the W3C ban on host
   lifted for that key only). The documents claimed once the processor
   passes them: test509, test510, test518, test519, test520, test522,
   test531, test532, test534, test567, test577. test201 leaves the
   exclusions and enters the corpus unclaimed (an ordering property no
   processor can meet).

The record also decides each contract gap the spike found (its
decision 8 table, with a column saying a session that registers nothing
sees no change for any of them) and dispositions each edge case the
spike listed (its decision 9 table).

Provenance

• The placement, both type strings with one location, and a host
  declaration in the case schema as a closed-set growth: ruled by the
  operator, 2026-09-29.
• No req dependency, with an OTP :httpc default transport: the
  spike's finding (st-lhwd), recorded here. It replaces the earlier plan
  of an optional req dependency; the reason is ADR-0062 fact 2, which
  the record answers by adding no optional dependency.
• The record's own engineering choices, each with its reason in the
  record: the optional ioprocessors_entry/2 over the alternative the
  spike named; a {module, opts} registration value over application
  environment; no effect field for params versus content; the processor
  reporting its own failed delivery through failed_send/3; the text
  rung for inbound form values; the key name event_io_processors and
  the per-key lift of the W3C ban.
• ADR-0075's Status line declares that it amends ADR-0069 decision 2 and
  ADR-0070 decision 5 in part. Neither file's decisions are edited here;
  ADR-0069 gains only the foot Note.

Direction check (in-turn)

Every claim ADR-0075 and the ADR-0069 Note make about code on main
was read at dc1900d0 (which is origin/main at this PR's base) and
cites by anchor: Statifier.Send.Processor's ioprocessors_entry/1
callback and moduledoc, Statifier.Send.Types.from_send_types/1,
Statifier.Evaluator.SystemVariables.initial/3,
Statifier.Machine.Content.Send's data/3, Statifier.EventData.coerce/1,
Statifier.Session's perform_instruction/3 and failed_send/3 (a
cast), t:Statifier.Session.Effects.instruction/0,
conformance/schema/case.json's W3C allOf branch,
tools/corpus/scxml_w3/exclusions.exs, the Basic HTTP templates in
conf_predicator.xsl, and the corpus files for the test199, test201 and
statifier-suite facts in the Note. Spec text is quoted from the local
spec cache. No Application environment read exists under lib/
(grepped). git diff origin/main -- docs/adr/ shows zero removed
lines: the Note is appended at the foot of ADR-0069 and the README gains
one row.

Gate

mix gate.verify on the committed tree, quoted whole:

Running quality checks...

✓ Format: No changes needed (369ms)
✓ Compile: dev + test compiled (warnings as errors) (342ms)

Running analysis stages in parallel...

○ Gettext: skipped (:gettext not installed)
○ ADR judge: skipped (disabled in .quality.exs)
✓ Doc links: 37 links checked (53ms)
✓ Dependencies: No unused dependencies (893ms)
✓ ADR guard: No likely ADR violations (1.4s)
✓ Sobelow: No findings (2.2s)
✓ Gate guard: No unjustified gate changes (2.2s)
✓ Doctor: Passed (3.9s)
✓ Conformance corpus: Passed (5.2s)
✓ Docs: No warnings (6.6s)
✓ Credo: No issues (10.0s)
✓ Dialyzer: No warnings (12.0s)
✓ Regression ratchet: Passed (15.7s)
✓ Tests: 3,067 of 3,067 passed, 96.3% coverage (19.9s)

✓ All quality checks passed!
Full gate green: scope all, no profile, 16 stages considered.
Not checked by this project at all: Gettext (:gettext not installed), ADR judge (disabled in .quality.exs)

mix adr.check: "No likely ADR violations". mix quality --profile merge (the ADR judge step): green; the judge skipped because no file in
the diff is in a judged ADR scope. The commit was made on the tree the
gate ran green on, with the repository gate lock and a machine slot held
across both.