fix: server boot-gate CI + QUICKSTART refutation demo + SPDX headers (#262)

## Summary

- **CI boot-gate** (`ci(server)`): new
`.github/workflows/server-boot-gate.yml` that builds the crate, starts
the server, and confirms `/api/health`, `/api/provers`, and a full
session round-trip — closes the gap between "compiles" and "actually
boots"
- **QUICKSTART demo fix** (`docs(quickstart)`): demo was a satisfiable
assertion (`(> x 0)`) which correctly returns `success:false`; replaced
with a refutation example (`(and (> x 0) (< x 0))` — unsat) that returns
`success:true` with explanatory comments on the refutation convention
- **SPDX headers**: fixed `QUICKSTART-USER.adoc` (stray
`(hyperpolymath)` in copyright text broke the pre-commit hook) and added
missing `SPDX-FileCopyrightText` to `src/interfaces/rest/main.rs`

## Test plan

- [ ] CI: `server-boot-gate` workflow runs green on this branch
- [ ] Manual: `cargo run -p echidna -- server --port 8081`, then run the
curl in the updated QUICKSTART → `{"success":true,"goals":0,...}`
- [ ] Pre-commit hook passes on all staged files

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

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: hyperpolymath

QUICKSTART-USER.adoc

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MPL-2.0
-// Copyright (c) 2026 Jonathan D.A. Jewell (hyperpolymath) <j.d.a.jewell@open.ac.uk>
+// SPDX-FileCopyrightText: 2026 Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>
 = ECHIDNA: Quickstart Guide
 :toc: macro
 :icons: font
@@ -120,12 +120,16 @@ curl http://127.0.0.1:8081/api/health
 # List Tier-1 core provers
 curl http://127.0.0.1:8081/api/provers
 
-# Prove a satisfiable SMT-LIB goal (Z3)
+# Refutation demo (Z3) — asserts a contradiction, expects success:true
+# ECHIDNA proves by refutation: success ⟺ unsat.
+# A satisfiable assertion correctly returns success:false (assert the
+# negation of your goal to check a validity obligation instead).
 curl -X POST http://127.0.0.1:8081/api/prove \
   -H 'Content-Type: application/json' \
   -d '{"prover":"Z3",
-       "content":"(set-logic QF_LIA)\n(declare-const x Int)\n(assert (> x 0))\n(check-sat)\n",
+       "content":"(set-logic QF_LIA)\n(declare-const x Int)\n(assert (and (> x 0) (< x 0)))\n(check-sat)\n",
        "timeout":30}'
+# Expected: {"success":true,"goals":0,"message":"Proof verified successfully"}
 ----
 
 The full route table is in `src/rust/server.rs`. Separate

src/interfaces/rest/main.rs

@@ -1,3 +1,4 @@
+// SPDX-FileCopyrightText: 2025 Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>
 // SPDX-License-Identifier: MPL-2.0
 // ECHIDNA REST API Server