W14: fix C22/K104 — harness logs IO before invoking the mock (+3 pins)

The interceptor appended the IO-log entry only after the mock returned, so
a throwing mock left no entry and error-path tests falsely reported "never
invoked" through assert-io-called/count (hosts.md C22, core.md K104).

spec/harness.sx make-interceptor now appends {:args :result nil :op}
BEFORE invoking the mock and updates :result in place via dict-set! on
return. This is W14-owned test infrastructure (PLAN.md W14 approach item
4), not a semantics edit.

Pins: suite gate-C22-throwing-mock-logged (throwing mock leaves an entry
with pending result; happy path updates the result; mixed throwing +
successful sequence counts all calls). Harness self-suite (15 tests) and
test-relate-picker (the only other harness consumer) verified green;
285/0 on the pins run.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

plans/agent-briefings/sx-gate-loop.md

@@ -69,7 +69,8 @@ Pin each confirmed-and-fixed finding with a minimal repro. Add suites to
 - [x] K19 — harness/runtime parity pinned (`scripts/test-harness-parity.sh`:
       drives mcp_tree sx_eval over JSON-RPC vs fresh sx_server over epoch,
       12-probe battery from the finding, errors compared by message)
-- [ ] C22/K104 — harness logs IO *before* invoking the mock (throwing-mock pin)
+- [x] C22/K104 — FIXED harness (spec/harness.sx make-interceptor: log entry
+      appended before the mock runs, :result updated via dict-set!) + 3 pins
 - [ ] C21 — real perform/suspend mode in harness
 - [ ] C23 — adapter-dom render-output tests
 
@@ -86,6 +87,19 @@ Pin each confirmed-and-fixed finding with a minimal repro. Add suites to
 
 ## Progress log (newest first)
 
+- 2026-07-04 — **C22/K104 throwing-mock fix + pins (item C.2)**. First
+  actual FIX of the loop — in scope because spec/harness.sx is W14-owned
+  test infrastructure (PLAN approach item 4 assigns "log IO before invoking
+  the mock" to W14). TDD: reproduced pre-fix (caught error, 0 log entries),
+  then restructured `make-interceptor` to append the entry BEFORE the mock
+  runs (`:result nil` while pending, `dict-set!` in place on return).
+  Verified: throwing mock leaves entry, happy path updates result, mixed
+  sequence counts all 3. Added suite `gate-C22-throwing-mock-logged`
+  (3 tests). Harness self-suite (15) + test-relate-picker (only other
+  harness consumer) green; 285/0 pins run. Tooling notes: replace/insert
+  tools take `new_source` (not `replacement`); find_all paths still
+  disagree with read_subtree/replace_node on define-library files —
+  sx_write_file remains the reliable route. Test-infra-only.
 - 2026-07-04 — **K19 harness-parity pin (item C.1)**. Authored
   `scripts/test-harness-parity.sh`: drives `mcp_tree.exe` `sx_eval` with
   raw JSON-RPC over stdio and a fresh `sx_server.exe` over the epoch