fix(ci): idris2-abi install via apt + tag-archive fallback — replaces 404 URL (#168)

## Summary

- `Type-check Idris2 ABI definitions` has failed on every run since at
least 2026-05-18 (5+ consecutive `failure`s in `gh run list --workflow
"Idris2 ABI Type-Check" --branch main`) because `curl -fsSL
https://github.com/idris-lang/Idris2/releases/download/v0.8.0/Idris2-0.8.0-Linux-x86_64.tar.gz`
returns HTTP 404.
- Root cause: Idris2 v0.8.0's release-asset array is empty — the project
did not publish a prebuilt Linux tarball for that tag. The
release-download URL therefore 404s. The same constraint is already
documented inline in `live-provers.yml:184-201`, which uses a working
install pattern.
- Fix: port the live-provers pattern into `idris2-abi-ci.yml`. Prefer
Ubuntu 24.04 universe `idris2`; fall back to a source build from
`archive/refs/tags/v0.8.0.tar.gz` (the archive API resolves for any
tag).
- Side cleanup: drop the `IDRIS2_PREFIX: /opt/idris2` env hint on the
Type-check step — neither install path lands binaries at `/opt/idris2`,
so it was steering Idris2 at a non-existent prefix.

## Why this matters now

This is the upstream-source fix the merge ruleset asks for. Removes the
baseline red so the rest of the queue (including #166's TacticRecord
round-trip lemmas, which this job will now exercise) can land without
`--admin`. Stops a 2-week-old failed-installation message from masking
any future regression in `src/abi/`.

## Test plan

- [x] `python3 -c "import yaml; yaml.safe_load(...)"` parses OK
- [x] Local check (Idris2 0.8.0 + base) succeeds on the same
`src/abi/echidnaabi.ipkg` with no `IDRIS2_PREFIX` override
- [ ] `Type-check Idris2 ABI definitions` job → success on this PR

Refs the 2026-05-18 → 2026-05-30 consecutive curl(22) run history.

Author: hyperpolymath

.github/workflows/idris2-abi-ci.yml

@@ -28,19 +28,29 @@ jobs:
 
       - name: Install Idris2
         run: |
-          # Install Idris2 from official release
-          curl -fsSL "https://github.com/idris-lang/Idris2/releases/download/v0.8.0/Idris2-0.8.0-Linux-x86_64.tar.gz" \
-            -o /tmp/idris2.tar.gz
-          sudo mkdir -p /opt/idris2
-          sudo tar -xzf /tmp/idris2.tar.gz -C /opt/idris2 --strip-components=1
-          echo "/opt/idris2/bin" >> "$GITHUB_PATH"
-          export PATH="/opt/idris2/bin:$PATH"
-          export IDRIS2_PREFIX="/opt/idris2"
+          # Idris2 v0.8.0 has no pre-built Linux release binary — the
+          # release-assets array on https://api.github.com/repos/idris-lang/Idris2/releases/tags/v0.8.0
+          # is empty, so the legacy `releases/download/.../Linux-x86_64.tar.gz`
+          # URL returns HTTP 404 and breaks this job on every push.
+          # Match the live-provers.yml install pattern: prefer the Ubuntu
+          # 24.04 universe `idris2` package; fall back to a source build
+          # from the archive tag tarball (the archive API resolves for
+          # any tag, unlike release-asset URLs).
+          set -euo pipefail
+          sudo apt-get update -qq || sudo apt-get update -qq --fix-missing
+          sudo apt-get install -y --fix-missing idris2 2>/dev/null || {
+            IDRIS2_VER="v0.8.0"
+            sudo apt-get install -y --fix-missing chezscheme make libgmp-dev
+            curl -fsSL --max-time 300 --retry 2 \
+              -o /tmp/idris2.tar.gz \
+              "https://github.com/idris-lang/Idris2/archive/refs/tags/${IDRIS2_VER}.tar.gz"
+            mkdir -p /tmp/idris2-src
+            tar xzf /tmp/idris2.tar.gz -C /tmp/idris2-src --strip-components=1
+            (cd /tmp/idris2-src && make bootstrap SCHEME=scheme && sudo make install PREFIX=/usr/local)
+          }
           idris2 --version
 
       - name: Type-check all ABI modules via ipkg
-        env:
-          IDRIS2_PREFIX: /opt/idris2
         run: |
           cd src/abi
           echo "=== Type-checking ECHIDNA ABI package (16 modules) ==="