main does not build on OTP 26 while README and the AI install guide promise it #97

main cannot build on OTP 26, but the README and the AI installation guide
both promise OTP 26 as the floor. An AI or a stranger following the documented
prerequisites on OTP 26 gets a compile failure, not a working install — so this
bears directly on the "install hyperpolymath's FoT please" goal.

Measured

On PR #96 (base 30a4a6be, the #93 merge commit) three CI cells fail:

Test (Elixir 1.15 / OTP 26)   FAILURE
Test (Elixir 1.16 / OTP 26)   FAILURE
Test (Elixir 1.17 / OTP 26)   FAILURE
Test (Elixir 1.19 / OTP 27)   SUCCESS

All three fail identically, in dependency compilation — before a single test runs:

src/cow_capsule.erl:18:2: function parse/1 undefined
src/cow_capsule.erl:30:2: spec for undefined function parse/1
** (Mix) Could not compile dependency :cowlib

Provenance — this is not PR #96

git diff 30a4a6be...HEAD -- elixir-mcp/mix.lock on PR #96 shows exactly one
line changed: gen_stage removed. PR #96 does not touch cowlib. The bump
landed with #93 and is already on main — #96 merely runs CI against it and
is the first PR to surface it.

Why it was not caught

The three OTP-26 cells are not in the required-status-checks list for main.
That list holds only CodeQL, CodeRabbit and governance / Code quality + docs,
so a red Test matrix cell cannot block a merge. #93 was merged with --admin.

The contradiction to resolve

• README.adoc:217-218 — "OTP 26 or newer"
• docs/AI_INSTALLATION_GUIDE.adoc:117 — "OTP 26 or newer"
• mix.lock — cowlib 2.20.0, which does not compile on OTP 26

Two coherent resolutions exist; this is a support-policy call, not a mechanical fix:

• A. Raise the floor to OTP 27. Drop the three OTP-26 matrix cells, change both
  documents to "OTP 27 or newer", and keep modern cowlib/bandit. Matches the
  toolchain the project actually targets (erlang 27.0, elixir 1.19.5-otp-27).
• B. Keep OTP 26 support. Pin cowlib back to a release that compiles on OTP 26
  (2.16.0 is proven to, on PR fix(ci): make the dogfood gate .deed-aware #94's base) and hold the matrix as it is.

Acceptance criteria

• The documented OTP floor, the CI matrix and what actually compiles all state
  the same thing. No document promises a version the build cannot satisfy.
• A green run exists on every OTP version the documents promise — the promise is
  backed by a measured cell, not an assumption.
• If the floor rises, README.adoc, docs/AI_INSTALLATION_GUIDE.adoc and the
  toolchain files are changed in the same commit, so no reader sees a stale floor.
• The relevant Test matrix cells are added to the required-status-checks list for
  main, or this issue records explicitly why a red build matrix is allowed
  not to block. A cell nobody is required to pass is a cell that will go red again.

Scope note

Filed per the owner's standing rule of 2026-09-15 — a new finding does not block a
merge; it becomes an issue with acceptance criteria. The defect is already on main,
so merging PR #96 neither introduces nor worsens it.