test(explore): #1500 regression fixtures for budget allocation (CG-6)

Two permanent fixtures pinning the failure mode from issue #1500 — explore
spending its byte envelope on files that merely name-collide with the query.
BOTH FAIL TODAY, by design: they document the bug and become the pass gate
for CG-10 (scoring) + CG-12 (proportional allocation).

__tests__/fixtures/payroll-go/ — a synthetic Go service mirroring the
reporter's shape: generated FKIT CRUD beside a hand-written payroll use-case,
entered from an HTTP route. Half the generated tree carries ORDINARY names
detectable only by their `// Code generated ... DO NOT EDIT.` header (the
#1500 case, and end-to-end cover for CG-5); `payrollpb/*.pb.go` covers the
path-detectable channel. BuildPayslip, Upsert and Store each exist twice,
generated and hand-written. cycle.go sits above the whole-file window so it
clips; the generated files sit below it so they ship whole.

Asking "how does payroll cycle create and calculate payslips?" — naming none
of the answering symbols — the generated CRUD delivers 57.4% of the envelope
against the hand-written layer's 25.6%, all of the latter domain types.
cycle.go is allocated the single largest slice (30.6%) and delivers ZERO: the
hard ceiling drops its whole section. runPayrollCycleAll, the hand-written
BuildPayslip and the real Upsert never reach the agent.

The second fixture is this repo, "how does explore allocate its output budget
across files", where scripts/agent-eval/*.mjs take 71.8% against tools.ts's
18.5% despite scoring 4.6x lower. It reads the live index, so its assertions
are relative rather than fixed percentages.

- scripts/agent-eval/probe-allocation.mjs — per-file budget-share probe,
  driving the CG-4 diagnostic through a JSONL sidecar so it measures the
  shipping allocator. Fixture entries are hermetic (copy + re-index per run,
  verified byte-identical across runs); exits 1 while any assertion fails.
- scripts/agent-eval/allocation-fixtures.json — both fixtures declared, with
  the 2026-08-03 baselines.
- __tests__/explore-allocation-1500.test.ts — fixture-shape assertions green
  today; the allocation assertions held as `it.fails` so the suite stays green
  while the bug is open and goes RED the moment it is fixed.

Also documented and deliberately left unfixed: runPayrollCycleAll's
`s.store.Upsert` edge resolves to the GENERATED Store.Upsert, not the
hand-written one — same-name method resolution across two packages picks the
wrong receiver. It is upstream of the allocation bug, so it belongs with
CG-10's scoring work.

Refs #1500

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

__tests__/fixtures/payroll-go/internal/gen/fkit/timesheet/timesheet.go

@@ -0,0 +1,76 @@
+// Code generated by fkit v3.11.0. DO NOT EDIT.
+//
+// Source: schema/timesheet/timesheet.fkit
+// Regenerate with: go run ./tools/fkitgen ./schema/timesheet
+
+package timesheet
+
+import (
+	"context"
+	"database/sql"
+	"time"
+)
+
+// TimesheetRow is the generated row type for table timesheet.
+type TimesheetRow struct {
+	ID         string
+	CycleID    string
+	EmployeeID string
+	Units      int
+	Approved   bool
+	ApprovedAt time.Time
+	CreatedAt  time.Time
+	UpdatedAt  time.Time
+}
+
+// TimesheetCreateRequest is the generated create payload for table timesheet.
+type TimesheetCreateRequest struct {
+	CycleID    string `json:"cycleId"`
+	EmployeeID string `json:"employeeId"`
+	Units      int    `json:"units"`
+}
+
+// CreateTimesheet inserts one timesheet row.
+func CreateTimesheet(ctx context.Context, db *sql.DB, req TimesheetCreateRequest) (TimesheetRow, error) {
+	const q = `INSERT INTO timesheet (cycle_id, employee_id, units) VALUES ($1, $2, $3) RETURNING *`
+	return scanTimesheet(db.QueryRowContext(ctx, q, req.CycleID, req.EmployeeID, req.Units))
+}
+
+// GetTimesheet selects one timesheet row by primary key.
+func GetTimesheet(ctx context.Context, db *sql.DB, id string) (TimesheetRow, error) {
+	const q = `SELECT * FROM timesheet WHERE id = $1`
+	return scanTimesheet(db.QueryRowContext(ctx, q, id))
+}
+
+// ApproveTimesheet flips the approved column on one timesheet row.
+func ApproveTimesheet(ctx context.Context, db *sql.DB, id string) (TimesheetRow, error) {
+	const q = `UPDATE timesheet SET approved = true, approved_at = now() WHERE id = $1 RETURNING *`
+	return scanTimesheet(db.QueryRowContext(ctx, q, id))
+}
+
+// ListTimesheetsByCycle selects timesheet rows for one cycle.
+func ListTimesheetsByCycle(ctx context.Context, db *sql.DB, cycleID string) ([]TimesheetRow, error) {
+	const q = `SELECT * FROM timesheet WHERE cycle_id = $1 ORDER BY employee_id`
+	rows, err := db.QueryContext(ctx, q, cycleID)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var out []TimesheetRow
+	for rows.Next() {
+		var r TimesheetRow
+		if err := rows.Scan(&r.ID, &r.CycleID, &r.EmployeeID, &r.Units, &r.Approved,
+			&r.ApprovedAt, &r.CreatedAt, &r.UpdatedAt); err != nil {
+			return nil, err
+		}
+		out = append(out, r)
+	}
+	return out, rows.Err()
+}
+
+func scanTimesheet(row *sql.Row) (TimesheetRow, error) {
+	var r TimesheetRow
+	err := row.Scan(&r.ID, &r.CycleID, &r.EmployeeID, &r.Units, &r.Approved,
+		&r.ApprovedAt, &r.CreatedAt, &r.UpdatedAt)
+	return r, err
+}