fix(ffi): replace removed std.atomic.Mutex with std.Thread.Mutex

[hyperpolymath]

The failure

Zig FFI Tests has been failing on every PR:

src/catalogue.zig:16:22: error: root source file struct 'atomic' has no member named 'Mutex'

This was invisible until the repository's Actions policy was fixed — the workflow
was startup_failureing before any job ran, so the compile error never surfaced.

The cause

std.atomic.Mutex was removed from Zig's standard library. Nine files in
ffi/zig/src/ still carried a byte-identical hand-rolled wrapper around it
(all nine hash to the same value — one template copied nine times):

const Mutex = struct {
    state: std.atomic.Mutex = .unlocked,
    pub fn lock(m: *Mutex) void {
        while (!m.state.tryLock()) std.atomic.spinLoopHint();
    }
    pub fn unlock(m: *Mutex) void { m.state.unlock(); }
};

The fix

The wrapper's public surface — lock() / unlock(), instantiated as
var mutex: Mutex = .{} — is already exactly std.Thread.Mutex's. So the whole
struct collapses to a one-line alias, with no call-site changes:

const Mutex = std.Thread.Mutex;

Two things worth noting:

• It's a behavioural improvement, not just a compile fix. The wrapper busy-waited
  via spinLoopHint, burning a core under contention. std.Thread.Mutex parks the
  waiting thread.
• It converges on the in-tree norm. 81 other files in this repo already use
  std.Thread.Mutex directly, so this removes a divergent copy rather than adding a
  new pattern.

Files: sla, catalogue, sdp, loader, verisimdb, guardian, community,
federation, coprocessor.

Verification

Run locally against the exact CI pin (zig 0.15.2, per zig-test.yml) — all four
targets the workflow runs:

Target	Result
zig build test	PASS — 13 + 3 tests passed
zig build readiness	PASS — 28/28 tests passed
zig build lib	PASS — 2/2 steps
zig build bench	PASS — 3/3 steps

Zero remaining code references to std.atomic.Mutex / std.atomic.spinLoopHint.

Note

.tool-versions pins zig 0.15.1 while zig-test.yml uses 0.15.2. Not changed
here, but the two should be reconciled.

[github-actions]

🔍 Hypatia Security Scan

Findings: 228 issues detected

Severity	Count
🔴 Critical	15
🟠 High	135
🟡 Medium	78

⚠️ Action Required: Critical security issues found!

View findings

[
  {
    "reason": "Action actions/checkout@v4 needs attention",
    "type": "unpinned_action",
    "file": "pages-deploy.yml",
    "action": "pin_sha",
    "rule_module": "workflow_audit",
    "severity": "medium"
  },
  {
    "reason": "Action actions/checkout@v4 needs attention",
    "type": "unpinned_action",
    "file": "pages.yml",
    "action": "pin_sha",
    "rule_module": "workflow_audit",
    "severity": "medium"
  },
  {
    "reason": "Action actions/checkout@v4 needs attention",
    "type": "unpinned_action",
    "file": "pages.yml",
    "action": "pin_sha",
    "rule_module": "workflow_audit",
    "severity": "medium"
  },
  {
    "reason": "Action actions/upload-pages-artifact@v3 needs attention",
    "type": "unpinned_action",
    "file": "pages.yml",
    "action": "pin_sha",
    "rule_module": "workflow_audit",
    "severity": "medium"
  },
  {
    "reason": "Action actions/deploy-pages@v4 needs attention",
    "type": "unpinned_action",
    "file": "pages.yml",
    "action": "pin_sha",
    "rule_module": "workflow_audit",
    "severity": "medium"
  },
  {
    "reason": "Issue in build.yml",
    "type": "missing_timeout_minutes",
    "file": "build.yml",
    "action": "flag",
    "rule_module": "workflow_audit",
    "severity": "medium"
  },
  {
    "reason": "Issue in pages-deploy.yml",
    "type": "missing_timeout_minutes",
    "file": "pages-deploy.yml",
    "action": "flag",
    "rule_module": "workflow_audit",
    "severity": "medium"
  },
  {
    "reason": "Issue in push-email-notify.yml",
    "type": "missing_timeout_minutes",
    "file": "push-email-notify.yml",
    "action": "flag",
    "rule_module": "workflow_audit",
    "severity": "medium"
  },
  {
    "reason": "Issue in scorecard-enforcer.yml",
    "type": "missing_timeout_minutes",
    "file": "scorecard-enforcer.yml",
    "action": "flag",
    "rule_module": "workflow_audit",
    "severity": "medium"
  },
  {
    "reason": "Issue in scorecard-enforcer.yml",
    "type": "scorecard_publish_with_run_step",
    "file": "scorecard-enforcer.yml",
    "action": "split_scorecard_publish_job",
    "rule_module": "workflow_audit",
    "severity": "high"
  }
]

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[github-actions]

🏁 path-claims bench

Commit 5b34e73

Numbers

path-claims bench  (node v22.23.1)

  scenario                                              iters       ms        ns/op          ops/s
  --------------------------------------------------------------------------------------------------------------
  register: 10 active claims, 3 new paths               50000 iters    179 ms      3.60 µs/op    278.1k ops/s
  register: 100 active claims, 3 new paths              20000 iters    314 ms     15.73 µs/op     63.6k ops/s
  register: 1000 active claims, 3 new paths              5000 iters    982 ms    196.57 µs/op      5.1k ops/s
  register: 100 active claims, 20 new paths              5000 iters    328 ms     65.66 µs/op     15.2k ops/s

  pathsOverlap: deep diverge at segment 4             1000000 iters    141 ms     141.2 ns/op     7.08M ops/s
  pathsOverlap: short prefix match                    1000000 iters    115 ms     115.1 ns/op     8.69M ops/s

  refresh (existing claim)                             100000 iters     10 ms     103.7 ns/op     9.64M ops/s
  list (100 active claims)                              50000 iters    264 ms      5.29 µs/op    189.2k ops/s

  (Bench numbers depend on host; use deltas across commits, not absolute values.)

Host-dependent — compare deltas across commits, not absolute values.