test(consensus): make path resolution independent of cwd (CI gate fix)

GitHub Actions CI (workflow 'Build All Platforms', run #29559727754)
flagged test-linux-unit and test-linux-sanitizers as failing. The CI
runs the test binary via 'ctest --output-on-failure' from build/, but
the consensus_safety_tests static-source grep tests called
readEntireFile('src/main.cpp') with paths resolved relative to CWD.

With CWD = build/, those paths did not exist; the tests failed with
'critical check !src.empty() has failed'. Same root cause for the
staking_tests::is_staking_safe_is_continuous_not_one_shot test which
opened 'src/miner.cpp' by raw __FILE__ slicing.

Specifically:
  consensus_safety_tests.cpp: 9 failures across
    convergence_rejects_below_hardened_checkpoint, hardened_checkpoint_init_is_startup_only,
    above_checkpoint_greatest_trust_wins, getheaders_recovers_via_genesis_when_locator_disjoint,
    getheaders_recovers_via_checkpoint_when_locator_has_it, reorg_guard_fails_closed_when_checkpoint_pointer_null,
    reorg_guard_offbyone_hardening, hardened_checkpoint_no_rogue_guard_in_other_files
  staking_tests.cpp: 1 failure in is_staking_safe_is_continuous_not_one_shot

Note: my pre-merge '280/280 tests pass' claim was based on running the
test binary directly from the repo root, where 'src/' resolves
trivially. ctest is the canonical CI invocation. This CI run was the
first time we exercised it.

Fix:
- Add findProjectRootFromHere(__FILE__) helper that:
  (1) prefers an absolute path in __FILE__ (/foo/bar/src/test/...),
  (2) falls back to a build-dir-relative anchor (./src/test/... or
      bare src/test/...) when cmake+ninja produces those,
  (3) defends against ctest's CWD=build/ by walking up from CWD
      looking for the canonical src/checkpoints.cpp sentinel.
- Apply uniformly in consensus_safety_tests.cpp (helper + 1 rogue-guard
  walker that builds project-root-relative paths before comparing
  against allowed_files) and staking_tests.cpp (mirrored helper).
- Strict-mode BOOST_REQUIRE_MESSAGE failure paths now include the
  resolved path so the next person debugging this hits the issue
  immediately.

Verified: 'cd build && ctest --output-on-failure' now reports 0
failures across all 4 ctest projects (triangles_unit_tests,
chaindb_equivalence_tests, snapshotnet_tests, chaindb_runtime_tests).
Direct 'test_triangles' invocation still works for ad-hoc checks.

src/test/staking_tests.cpp

@@ -3,12 +3,48 @@
 
 #include <boost/test/unit_test.hpp>
 
+#include <filesystem>
+#include <vector>
+
 #include "../main.h"
 #include "../kernel.h"
 
 extern unsigned int nStakeMinAge;
 extern unsigned int nStakeMaxAge;
 
+// Resolve the project root from this test file's __FILE__.
+// See consensus_safety_tests.cpp::findProjectRootFromHere for the full
+// rationale — the version here is kept in lock-step so that local
+// `ctest --output-on-failure` runs from build/ succeed.
+static std::string findProjectRootFromHere_staking(const std::string& here)
+{
+    namespace fs = std::filesystem;
+    std::string h = here;
+    while (h.size() >= 2 && h[0] == '.' && h[1] == '/') h.erase(0, 2);
+    size_t abs_pos = h.rfind("/src/test/");
+    if (abs_pos != std::string::npos) {
+        std::string root = h.substr(0, abs_pos);
+        if (!root.empty()) return root + "/";
+    }
+    size_t rel_pos = h.rfind("src/test/");
+    if (rel_pos != std::string::npos) {
+        std::string prefix = h.substr(0, rel_pos);
+        fs::path candidate;
+        if (prefix.empty()) candidate = fs::current_path();
+        else candidate = fs::path(prefix);
+        if (fs::exists(candidate / "src" / "checkpoints.cpp"))
+            return candidate.string() + "/";
+    }
+    fs::path cur = fs::current_path();
+    for (int i = 0; i < 8; ++i) {
+        if (fs::exists(cur / "src" / "checkpoints.cpp"))
+            return cur.string() + "/";
+        if (cur == cur.root_path()) break;
+        cur = cur.parent_path();
+    }
+    return "./";
+}
+
 BOOST_AUTO_TEST_SUITE(staking_tests)
 
 // --- GetWeight: coin age weight calculation ---
@@ -354,14 +390,17 @@ BOOST_AUTO_TEST_CASE(is_staking_safe_is_continuous_not_one_shot)
     // wait loop. Post-fix must NOT have the fTryToSync flag at all.
     //
     // Use __FILE__ to find the repo root so the path resolves regardless
-    // of the build directory or test runner cwd.
-    std::string here = __FILE__;
-    size_t pos = here.rfind("/src/test/");
-    BOOST_REQUIRE(pos != std::string::npos);
-    std::string miner_src_path = here.substr(0, pos) + "/src/miner.cpp";
+    // of the build directory or test runner cwd. The resolver tolerates
+    // both absolute paths and the bare-rel or "./"-rel forms cmake+ninja
+    // sometimes bake in, and falls back to walking up from CWD looking
+    // for src/checkpoints.cpp.
+    std::string root = findProjectRootFromHere_staking(__FILE__);
+    std::string miner_src_path = root + "src/miner.cpp";
 
     FILE* f = fopen(miner_src_path.c_str(), "r");
-    BOOST_REQUIRE(f != nullptr);
+    BOOST_REQUIRE_MESSAGE(f != nullptr,
+        "Could not open '" + miner_src_path + "' — repository root resolution is "
+        "broken; ctest from build/ would also fail.");
     fseek(f, 0, SEEK_END);
     long nSize = ftell(f);
     fseek(f, 0, SEEK_SET);