fix(ci): use the pinned Julia 1.11.5 in the ABI-FFI gate (#175)

## Summary

Follow-up to **#174**. That PR's `Install Julia` step pointed
`$GITHUB_PATH` at `/tmp/julia-1.11.5-linux-x86_64/bin`, but the official
Julia tarball extracts to `julia-1.11.5/`. The path never existed, so
the `ABI ↔ FFI structural conformance` gate silently ran on the runner's
**pre-installed** Julia and the 1.11.5 pin was a no-op — green today,
but wasteful (a ~150 MB download that's never used) and not reproducible
(it would break if the runner image ever drops Julia).

## Changes

- **`.github/workflows/abi-ffi-gate.yml`**
- `$GITHUB_PATH` → `/tmp/julia-1.11.5/bin` (the actual extracted
directory), so the pinned Julia is the one on `PATH`.
- `julia --version` echoed in the gate step, so the CI log shows the pin
is in effect (and any future regression is visible).

## RSR Quality Checklist

### Required

- [x] Tests pass — gate behaviour unchanged (verified against the same
tree under julia 1.11.5: `OK — 24 ABI functions exported, 8 result codes
match`)
- [x] Code is formatted — YAML validated locally
- [x] Linter is clean
- [x] No banned language patterns
- [x] No `unsafe` blocks — n/a
- [x] No banned functions
- [x] SPDX header unchanged
- [x] No secrets, credentials, or `.env` files

### As Applicable

- [x] ABI/FFI gate behaviour preserved (this only fixes *which* Julia
runs it)
- [ ] State files / CHANGELOG — no change

## Testing

The extracted directory name was confirmed directly: `tar -tzf
julia-1.11.5-linux-x86_64.tar.gz | head -1` → `julia-1.11.5/`, and
`/tmp/julia-1.11.5/bin/julia` is the real binary. The gate itself is
unchanged from #174 and still reports `ABI-FFI GATE: OK (verisimiser) —
24 ABI functions exported, 8 result codes match`.

## Screenshots

n/a (workflow only)

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🤖 Generated with [Claude Code](https://claude.com/claude-code)


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_Generated by [Claude
Code](https://claude.ai/code/session_01JdqVWGSSv36Ph8ZWvizGMp)_

Co-authored-by: Claude <noreply@anthropic.com>

Author: hyperpolymath

.github/workflows/abi-ffi-gate.yml

@@ -25,9 +25,11 @@ jobs:
         run: |
           curl -fsSL https://julialang-s3.julialang.org/bin/linux/x64/1.11/julia-1.11.5-linux-x86_64.tar.gz -o /tmp/julia.tar.gz
           tar -xf /tmp/julia.tar.gz -C /tmp
-          echo "/tmp/julia-1.11.5-linux-x86_64/bin" >> "$GITHUB_PATH"
+          echo "/tmp/julia-1.11.5/bin" >> "$GITHUB_PATH"
       - name: Run ABI-FFI gate
-        run: julia scripts/abi-ffi-gate.jl
+        run: |
+          julia --version   # confirms the pinned 1.11.5 is on PATH, not the runner default
+          julia scripts/abi-ffi-gate.jl
 
   zig-build:
     name: Zig FFI builds + tests (Zig 0.14.0)